This is my first build and i've been playing Singed for a long time now and have found that all of the builds for him on mobafire were good enough to do well but just didn't fit exactly what i wanted to do with him so here is my approach to playing Singed. If you want me to improve this guide please leave a comment about what you think i should change and how as well as leave a vote to let me know how good you think the guide is at this point in time. If you'd like to play a few games with me just add me in game and send me a message sometime. I would really like it if someone would like to play Singed for me.
(my name is Billeh Billy).

The Pros and Cons to Singed?

Pros:

• Deadly to pursue
• Very Tanky
• Incredibly mobile
• Hard to run from

Cons:

• Few CC's
• "easy" to avoid damage source
• Everyone wants to kill you
• "easy" to counter

For my marks I use 9 Greater Mark of Magic Penetration because everyone gets some sort of Magic Resistance and the approximate 10% Magical Penetration helps to bypass some of this so I don't feel the need to get an item with Magic Penetration. For my seals I use 9 Greater Seal of Armor to give me an early game armor boost to help survive against one of the biggest threats early game... Auto Attacks... For my glyphs I use 9 Greater Glyph of Scaling Magic Resist I really like the extra magic resist for late game because those burst champions need to have their damage kept in check. For my Quints I have found that a lot of quints work really well with Singed and that you can really use any you want. I just use 3 Greater Quintessence of Knowledge because the extra Mana which with his passive Empowered Bulwark becomes extra health is awesome for keeping yourself alive longer and being a bigger threat to the enemy team; however you could use Greater Quintessence of Ability Power or Greater Quintessence of Health or even Greater Quintessence of Movement Speed. These are all viable choices in my opinion.

Masteries in my opinion are used to help you out early game. So for my masteries I usually go 21-0-9 in which I get masteries that help my magic damage increase in the offense pool and in the utility pool I get masteries that help me with movement speed and give me more mana. Sometimes I find that my team needs me to be able to stay in fights and take all of the damage for them rather than simply run through trying to do as much damage as I can I will go 0-21-9 with the same utility masteries but masteries in the defensive pool that I feel will help me survive all sorts of damage.

I love to start with boots and 3 health pots... this is really personal preference and you can start with whatever you want to accomplish your goals.
You may ask, why not get more mana and in turn health with singed? and my answer is that with this build you will be incredibly tanky early and don't need all of the extra health once you start steamrolling with singed and get ahead with levels and items they won't do enough damage to scare you anyway. I started using this setup one day when i was reacting to a AD heavy team who had a little of an upper hand on me early game so i got a chain vest to counter them... well the only useful thing to build this into is the Zhonya's Hourglass because I like to play Singed as a carry... after I got my hourglass the team had a Morgana that started to have an upper hand on me so I got a Force Of Nature, I really love the Force of Nature on Singed and consider it to be one of his bread-and-butter items like the Rod of Ages. Once this game got to late game my team was having a really hard time killing the enemy Malphite who had build a lot of health so i decided to build a Deathfire Grasp which is one of my favorite items up to date... this was to shoot him with to initiate so he would have a "handicap" of only about 2/3 health. The Rabadon's was just for overkill... Don't think that the items in my guide are a strict code to stick to, make sure you get a feel for the enemy team and who is doing the most damage to you and what type of damage they are doing and try to counter this with an item that gives AP like I listed above.
Make sure that if you get items with an active like a Deathfire Grasp that you use this active as often as possible because they are almost always on a really short cooldown.
I will add more to this section later if you guys would like a better explanation.

I use this skill sequence to accomplish my goals... I like to kill so kill I do. It's always fun to be able to fling the enemy Viegar and have him start the fight with 2/3 health... Also, always remember to try and get people to run through your puddle twice by throwing it in from of them and flinging them back onto it.
I don't get more points into my puddle until late game because i want to do more damage and the pudde simply doesn't do any damage... Also, i have found that the slow at level 1 is great enough to let me catch up to almost any enemy.
Will add more in this section if you guys would like more of an explanation.

Summoner spells are all preference... I really love flash but for me Ghost works so much better for singed because he is tanky enough to have the extra time to run away from 3 or even 4 people and still survive... I also get Cleanse because this helps me in running away from 4 or more people with lots of CC. Really you can bring whatever summoner spells you wish to bring. A good summoner spell to take if you find you aren't doing quite enough damage is Ignite... it synergizes really well with your poison.
Use the summoner spells you know your enemy will hate you for getting.
I will add a better explanation to this later if you'd like.

Singed is great at supporting his team while still doing really good damage. As Singed always try to save your teammates if you guys are being chased throw down a Mega Adhesive behind your buddies to keep them from getting killed, this will make them love you and make the enemies hate you. A good Singed will be in the back of his group of friends while they are running away to try and do some damage as well as absorb any damage the enemy might be throwing at your teammates to kill them.

When dual laning with Singed make sure your teammate knows when yo are going to fling an enemy... You don't want them missing skill shots because of you.

Skill Sequence >< (because something messed up while I was making this)
Pretty Pictures!
Gameplay videos
Go more in depth in the guide...