Singed is by far one of my favorite characters, and the early game is crucial for Singed as well as his team. It's best to try to team up with a fighter that knows what they're doing, my partner of choice is Xin Zhao. Buy your items (boots, 2hp pots, 1mana pot) and quickly make your way into the first set of brush, and wait for them to come into yours. I always put my first point into Fling because from the getgo, fling does good damage, and puts them farther from their tower than they want.

If you don't get the early kills don't worry! Let their minions push out and wait in the brush. Don't worry about saving your pots, because you'll be killing before you know it. Wait for them to push out, then hit sprint and fling again, and hopefully get your first kill.

Get your poison at 2, Mega Adhesive (from here on out I'll call it goo) at 3, and then just start maxing your poison as quick as possible. The poison is Singed' number one assault in my opinion. Poison + Fling poisons them in mid air, then they have to run through it again to get back into position. With the magic penetration, you're going to be doing almost full damage with the poison, and it brings down everyone - Tanks and Carrys - as quick as lightning.

Conserve mana by just turning on and off your poison cloud where they're at, I don't think the poison stacks, so let them get the effect of it, hit the switch, then turn it back on once the effect is wearing off.

By now you're around level 6-10, and the laning phase is starting to fall out. As Singed, you have better map control than just about any other champion. Ghost + Insanity potion makes you run like a track star, and with your poison maxed out, you'll be able to make one loop around a set of minions and they'll all die just from the first poison cloud. You should have your Rod of Ages by now, and should be starting to help ganks. Play smart, hit your poison, fling them towards your team, poison the area you're standing on (the area they have to run back through) and let them run through your poison cloud.

Singed makes a great tank after you get your Guardian Angel because no one wants to attack the guy that's just going to come back to life. So pick off the weak one that's just behind the crowd, sprint + poison + fling, then drop your goo between yourself and the enemy members that turn around to try to save their friend, leave them a little poison, and watch your enemies fall.

After you get your Abyssal Scepter and Banshee's, you're sitting pretty with all the damage you do from your fling and poison cloud, and you're pretty well AP protected. It's time to decide for endgame what you need the most protection from, either AP or AD. So I usually choose between Thornmail, or Force of Nature. Or if you feel you only need a little more armor, get the Zhonya's Hourglass, which will add 100AP plus 50 armor.

Group up with your team and continue to poison throw their carry's backwards, separate with the goo, then poison everything in the area. Make sure to hit your Insanity potion right after you perform your fling, because most battles won't last 20 seconds. That way you can take the Fight or Flight option, and see where you need to go.

I like Magic Penetration runes the most, because I think that Singed' poison cloud is the most effective ability he has, however underrated it is. It allows you to last hit on people you're not standing by, and allows you to keep damaging your enemies while you're being chased.

I like the late game Mana Regen because it's nice to be able to turn on poison cloud and run around barely draining any mana at all.

And I have cooldown runes because it allows you to fling opponents constantly, both into the hands of your teammates, and away from your weaker-about-to-be-killed allies.

Sprint and Exhaust is always my favorite choice. I mean sprint is great for lane control, and chasing opponents down.

Exhaust is my favorite technique, because using your goo effectively is like having a second exhaust, so use it right after they get out of the goo for a second and great effect.

Use Fling to catch prey, and to throw enemies away from your allies when your allies are being chased.

Poison everything, when you're attacking, running, or just farming.

Use your ult to help with lane control, and to help you catch opponents.

Use your goo to slow down the enemy team from reaching a cut off ally, or to slow down a single target for the kill.

Pros

Tough
AP effective
Fast movement speeds
Tank
AoEDoT
Slows
Team Carry

Cons

Weaker at start
Not great against opponents with lots of CC (Alistar, and anyone else with knockup)
Not an easy character to learn

I like to play defensive masteries being a tanky character, but as opposed to going utility, I think Singed does much more damage going offensive and getting that +15% Magic Penetration.

I'll actively update as new things and ideas come along, if you want to bash, go ahead, but at LEAST give me some ways you think I could improve the guide. I don't know how to show my match history, but currently I'm averaging around 12-3-24 if that helps at all.

Thanks for reading, please give me your input!