Hello everyone, I've been playing singed for some time now, and I was hoping to help out all those who are just starting up or trying to improve on singed. Singed is a great tank and initiator, and most importantly a killer. Tanky dps/DoT characters are slightly overpowered in LoL in my opinion, I would say singed has an advantage over others, and hopefully you'll learn how to exploit it!

Why do you want to use Singed? Here's why:

Pros:
VERY tank late-game
Fling is great, high damage and helps the team so much
Poison allows you to kite, creating many opportunities to counter-kill a chasing opponent
Fast movement speed, feels good to walk fast
Singed pisses people off, use it to your advantage
Under the correct environment, Singed can carry like a boss!

Cons:
No ranged damaging abilities, goo only slows opponents down
Somewhat item dependent (Only if you fail at farming)
Cannot deal with physical dps's ALONE during late-game

For those who are too lazy to read up on the guide, heres how you should play singed:

NOTE: This will not teach you the tricks that help you get kills and juke like a god, so read up on the full guide if you have the time!

Items:

1. 
   Start with a Sapphire Crystal and 2 health potions
2. 
   If you can stay out until you have enough money for Rod of Ages, by all means rush it.
   Otherwise, get the standard boots somewhere in between.
3. 
   Get Sorcerer's Shoes, or if your team is doing EXTREMELY horrible, get Mercury's.
4. 
   I usually go with Force of Nature, but if the enemies are dealing ridiculous amounts of AD, go for some armor. I would suggest Thornmail.
5. 
   If the game isn't over by now, go for another tank item. Abyssal Scepter is highly recommended for its magic penetration, but if you aren't doing so great go for something with more survivability like Guardians Angel, or something against dps's like Frozen Heart.

Playstyle:

Early Game:
Level up without you or your lane partner dying.

Mid Game:
If team fights look like they are popping up, HELP OUT.
Otherwise farm up.

Late Game:
Initiate for the team! Singed has very high move speed, with his ghost+cleanse+ult+goo he should outrun anyone. Fling enemy squishies into your team and run through the enemy team. Usually I dont turn back since the entire enemy team will be on me, but if they go after your team like they should turn back to help with goo/fling/poison. Hopefully you'll end up killing a few, not dying, and win the game! gl hf :D

For Runes I go with:

Magic Pen Marks for more damage/kills
Dodge Seals which help with the mastery setup I have
CDR Glyphs for faster cooldowns which means more help to the team and kills
Movement Speed Quints to help you outrun your enemies

Feel free to switch up the runes however you wish, I have the dodge seals there because I don't focus as much on armor in my build, and this is my form of compensating for it with some dodge. However, if you play Singed the way I do you should be RUNNING ALL THE TIME, which means not that many autoattacks will hit anyways.

I go for 0/21/9 masteries, quite standard for any tank.

I DO go for Ardor because I'm selfish and like to get kills, but if you wish go ahead and add those wherever you like.

Perseverance is quite a useless mastery, Good Hands helps in those late-game situations where a couple of seconds could mean a comeback or defeat. If you prefer the insignificant amounts of regen Perseverance grants, go for it.

I get 1 point in Meditation because I have slight mana problems early on, you may want to get Awareness for the slight level advantage. Greed is almost useless imo, if you focused more on playing you can get one more last hit which lasts you an average of 200 seconds of this mastery. Although I am greedy as hell, getting neutral buffs does not help singed much, having a teammate who can utilize the buff much better will help me out anyways, help your team grab the buff and try to steal the 2 small ones around it :P

ABSOLUTE CORE ITEMS:
That's it, Rod of Ages is almost all you need to play well up until mid-late game. Of course, if you play him correctly and its a good game I've gotten RoA as soon as the 11 minute mark.

EVERYTHING Rod of Ages grants helps Singed out in some way.
HP? What tank doesn't need it?
MP? Good for poison, adds 1/4 of that to health as well!
AP? Adds to your poison and fling, awesome

I would recommend rushing this item, even before boots, if you had enough money. Keeping the ghost and ult up for situations where you need to walk fast will be important if you go with this.
I almost ALWAYS go with Sorcerer's Shoes, cleanse beats Mercury's by so much I don't see why you would have it for Singed. Even in losing games, if you played it safe you wouldn't be too far from a tower. Ghost + Cleanse is like a get out for free combo, and against bad players it can even net you a KILL and an escape.
After RoA, I'd say this is the most defining item for Singed. This item grants survivability and movement speed, both are great for Singed, as it makes chasing, juking, escaping all so much easier. I would only skip this item if the enemy team was ALL AD, something like Tryndamere, Master Yi, Udyr, which is ridiculous.

Extensions:
If everything's going along merrily, go ahead and grab this item. This item grants AP, MR AND MAGIC PENETRATION! I don't get this item earlier because Force of Nature is just too nice. Abyssal lets poison pack a harder punch, and fling is almost a nuke now, sweet!
I'm going to be honest here, Singed is not the best tank to play against physical dps characters. If enemy autoattacks are starting to get annoying, this is what you should consider.
I'd suggest thornmail because it's cheaper and hurts the enemies more when you get hit, which allows you to:
Pop ghost and cleanse and ult (whichever's available)
Run back if they are ranged
Fling their stupid champion for chasing you
Run again until fling is on cd again
Guardian Angel is a nice late game item, which lets you tower dive + fling knowing that you WILL be safe for a bit, so that your team will get the kill while you revive and run back out. It is also good to let enemies think they can kill you, which keeps your team safe while you run around getting enemies pissed off taking damage from both your poison and you teammates the whole time.

FOR THE LULZ:

I've had a couple of games where I was already legendary, where even if the entire enemy team focused me it would take too long for them to kill me. If games like that ever show up:
Fling will be a NUKE, doing 1/3 to 1/2 health on enemy squishies. Poison scales at 30% AP, which is decent considering it is multi-targetting and damage over time.

Items I don't use and why:
I don't think this is necessary if you have force of nature and cleanse. Banshee's gives less MR and more hp and mp, but Force of Nature gives movement speed which is extremely important for Singed.
Rylai's is nice to have at earlier levels, as it makes slowing down enemies so much easier. However, I would say RoA provides much better stats, as the effects of Rylai's are not as obvious late-game. I have tried Rylai's at different phases, and even the Catalyst+Rylai's build, but in the end I think it is better replaced with RoA + a tank item later on.
Building Singed this way, you will have Force of Nature and ghost. Using them correctly will allow you to catch up/run away from enemies easily enough, getting Boots of Swiftness and sacrificing Magic Pen/Magic Resist is not worth it.
Perhaps these are a viable choice under the correct context(Pro Trynd/Yi/Udyr), other than that the dodge provided by your masteries and runes should be sufficient.

This is it, ladies and gentlemen, the reason I felt like I had to write this guide. Hopefully some of you will read through the walls of text(I may add pictures if this guide becomes noticed) and get to play Singed LIKE A BOSS!

Note:

Don't feel bad for getting kills! Flinging someone for the kill is not so bad most of the time. If the kill would not have been possible without you, I think it is fully justifiable that you get the kill. Having someone say "wtf I just used my ult to kill u ks me" is just plain ******ed if it wasn't needed for the kill, learn to not waste your cd/mana.
HOWEVER, if you feel that one of your team members contributed the most to the kill, try not to ks(sometimes poison does it anyways, in which case it's not your fault).
In general though, it is better to be safe than sorry, killing someone fast so that he/she doesn't escape or before help arrives is better than getting no kill at all.

For some characters like Mord, if they are going to die it is best for them to stay and fight. IT IS NOT THE CASE FOR SINGED, staying alive for as long as possible will allow the most poison damage to set in, and being bait and outplaying the opponent is never bad. Don't overdo it, though, as there are times where I foolishly overestimated my survivability and got ruthlessly killed while trying to kite.

Placing your fling is utterly important, a Singed who sucks at flinging is like a gangplank who sucks at critting. Fling can be used to save your team by flinging your opponents away from a teammate who is being chased, it can be used to fling enemies into your team to ensure a quick kill, it can be used to juke, when you turn back to fling and run in a different direction.
Don't be too anxious to wait a little before you fling. It is better to fling an escaping enemy than to initiate with a fling that lands them at the same, or even less distance to safety. It may also be wise to consider your team's skillshots(if any), try to communicate with your team so that no skillshots are wasted because of the sudden change in positions.
If you know they are going to die(100% sure) from your poison after the fling, fling them and get the kill while your team is confused by the sudden change in positions!

Sometimes there will be an opponent who is clearly unskilled. Don't feel bad to make his/her life a living hell! Singed is a farmer, so even if you don't get a kill after sacrificing some farm remember a good wave or two will bring you back up.

Early Game:

Playing defensive is, believe it or not, the best way to get kills. Enemies may lower their guard thinking that they are dominating the lane, while they push towards your tower.

One typical combo to get an early game kill is to fling the enemy into your tower, then poison trail ahead of them. Keep in mind though, the tower WILL NOT attack the enemy champion if it is already hitting a minion. In order for this tactic to work, there must be no minions within tower range, or somehow the enemy champion damages you or your lane partner while within tower range(tower will immediately prioritize that champion).
Of course, flinging an enemy who has overextended is always a good decision, as it allows your fling, your poison, and your lane partner to harass the enemy while he runs back to safety.

During early game, I do not recommend damaging minions with poison. First of all, you will be spending a lot of mana. Secondly, your poison will not be strong enough to kill a minion quickly. If you are dominating the lane, or if the enemy is playing too defensively, zone them out of the xp range and focus on last hitting.
I will say that earning money early game is quite important, as getting the Rod of Ages makes Singed A LOT stronger. DO NOT risk you or your lane partner's life to get a kill/last hit though, it's almost never worth it(unless you were the one who was going to die anyways, sometimes a fling before dying can bring an unexpected kill). I would not recommend tower diving before level 9~10, but if they are ONE HIT away and you have ghost+cleanse, use it and fling the opponent to his/her death!


Mid Game:

At around level 9 with RoA. I usually start to roam a bit, going to lanes which are being pushed to see if there are any kills to be had.
Singed's ultimate is great both offensively and defensively:
While being chased, turn it on to deter opponents with your higher-damaging poison. If they are foolish enough to chase, fling them(into your tower) and run ahead of them, they should not be able to outrun you as you have increased movement speed/ghost.

*Trick: Against offensive enemies, you may run towards your tower to feign "escaping", while in fact you are waiting for the opportunity to run back and fling them into their deaths!
*Trick 2: Brushes are your friend! If the enemy is chasing and you think they can die from a fling > poison ahead of them combo, stay still RIGHT AFTER you enter the brush and fling them as they are about to enter. This will confuse them, giving you time to autoattack or position yourself for the kill! This trick has gotten me many, many kills while being ganked(even by more than 1 person), and the tactic lasts well into the late game.

Team fights should be popping up now and then, it is important that you do your part as an initiator! Stay in front of your squishies, and look for opportunities to fling! Tossing down a goo to slow enemies will also help a lot. Tower diving is relatively safe if you have your ghost+cleanse ready, so don't be scared to get hit a couple of times in order to get a good fling!
I usually run through the entire enemy team, allowing the poison to damage most of them. This also creates an awkward situation for enemies, as your team has "surrounded" them. Run ahead of them if they retreat, or close in and use your fling to deal some damage and kill(ONLY if your are doing well! Don't ks otherwise). I usually save up Mega Adhesive for sticky situations(hahaha, see what I did there?), where a well placed goo can get a kill or save your teammate.
If you know that you will not survive running through the enemies, you should run back past your team. This will make them think that you have retreated from battle, while in fact you are waiting for another opportunity to goo/fling/poison them again now that your ghost and ult are still up. Go in and tank the damage if your team is not doing well.

*Tip: Don't use ghost/cleanse/ult until you are decently close to your enemy! your sudden increase of speed will catch them off guard, allowing you to get that fling off early enough so they will be flung close to your team. It is up to you to use the goo before or after the fling.

*Tip 2: ISOLATE! Singed is so awesome not only because of how tank he is, or how much he can farm up, or how he can run as fast as Master Yi, but also because of his FLING. Isolating an enemy squishy and killing him/her fast can really shake the enemy morale, and doing this enough will get them to fear you!

*Tip 3: POISON ON AT ALL TIMES DURING A FIGHT! You should not have any mana problems by now. If you have run out of mana, you are either spending too much during other times, or you have been on the attack for way too long. A drawn-out offense only creates weakness within your team, call b and allow your ghost/cleanse to cd a bit.

Sometimes, SOMETIMES against weak opponents you can 1v2, even 1v3 them if you kite them correctly. Using the brush/fling combo and saving ghost/cleanse for bad situations allow you to run around relatively safely while the enemies are taking consistent poison damage. Getting a kill like that during a gank will deter you from future ganks. Also, it makes you feel quite godly.

While not much is going on, look for lanes where there are no enemy champions and their minions are pushing towards your lane. Either run to the fight or run ahead where the enemy minions are coming in from and farm the entire wave with your poison. This gives you quite a nice bit of farm, and will keep you ahead of the game. Be wary of ganks though, map awareness is very important.

Late Game:

I always hated reading stuff like "By now you should have at least a couple of kills...", sometimes the game is just going HORRIBLY and you are at 0/6/0. If that is the case, hopefully you've lasted till late game, where a good team fight can change the tides of the game! Playing as Singed, I love to "overextend" in order to get the other team to initiate, then pop ghost/cleanse and run back towards your team. The enemies will be pissed they didn't kill you, and if they chase into your poison and into your team, it's almost guaranteed that your team will be able to pick off one or two of the enemies. Let your equipment decide how much you should "overextend": if you are decently equipped rush in a little bit more, whereas in bad games only venture out a little.

In games where you have been doing well(I consider dying less than 5 times while having anything from 5 to 15 kills doing well), this is where you will shine! Singed is insanely tank late game, so don't be afraid to run past 2 tanks to fling the squishy characters.
Again, pretending to overextend while your team is ready does wonders. Chasing enemies often don't notice how much health they have lost from chasing your poison, and running back to fling them really scares them into forgetting to do something that could've saved their lives.

When the enemy team is aced or if there is only 1~2 enemies left alive, you may tank a tower while a dps on your team wrecks it. WATCH THE TIMER, these things, along with map awareness are things you must look out for while playing any hero.

I've been playing Singed for quite some time, and using my builds have often times brought on huge amounts of annoyance/suffering upon my opponents. For those who build Singed a different way, you should really try this build out as IT WORKS:

I know I am just starting up ranked play, and the teams are indeed quite noobish at times, but at least they are all level 30s and have some experience playing.
Any suggestions/criticisms are welcome, this is the first guide I've ever written.
I know the text is quite abundant, but I do believe there are little tricks and tweaks in there that if you pulled them off in a game you'll be feeling quite good about yourself :D
Good luck and have fun!