This Singed build concentrates mainly on giving your poison trail time to destroy the enemies health bars. The best way of doing this is to be able to tank other teams damage. Also once you have a Sunfire Cape if they get to close to you they start to take damage from that as well.

The set up of the runes can be able to last-hit minions at lower levels of Poison Trail, which can just take the health and leave minions' last hit for your own minions. Singed doesn't have trouble in-game in keeping alive and the runes are set up to take down your enemies.

The masteries are aimed defense and taking down your enemies as well. If you are soloing top then the Siege Commander Mastery can help make up for lower physical damage, it also means that your team can push turrets in a team fight with greater ease.
If you want to have different masteries that may buff your passive, give you greater mana regeneration or other things like that they would work just as well.

The items shown above are a fairly good guide for what you should be building Singed, however there are two possible differences. Instead of a Catalyst the Protector you can build the Sapphire Crystal into a Tear of the Goddess and from there an Archangels Staff.
The other deviation is if the other team has quite a lot of Magic Damage then you could change a Thornmail into an Abysmal Scepter or another type of Magic Resist item.

Also remember to bye wards if you are being ganked. I cannot stress enough how important wards can be with Singed.

The skill sequence above is fairly standard however you can max Mega Adhesive over Fling if you feel the slow is more important to that game.

The Summoner Spells i have on Singed are quite different to what is normally used. If you want to run a more standard summoner spell set-up then i would suggest Ghost and Teleport or any combination of the four Spells.
The reason I run Summoner Exhaust is because of the extra slow which stays to a particular target and because it slows the target and lowers their damage output, meaning that your team can approach a high damage target more easily.

A quick not on farming. Singed is one of the best farmers in the game, along with Amumu. the reason for this is that you just have to run around the minions and then, once Poison Trail is at a high enough level, you can then harass the enemy, while the minions behind you are dieing.
Singed is also very good at creating large minion hoards for your team, and getting rid of the enemies hoards. Simply run ahead of where your minions are fighting and get rid of the reinforcing wave of enemy minions. Your minions get a numbers boost and the enemies don't. They other good part of this is that Your minions stay at full health for longer.

The main idea of Singed is to destroy other team. The ways to accomplish this are to run circles around them, block there escapes with Mega Adhesive and Tower Dive with Insanity Potion.
Be warned however that the enemies carries and high damage champions will start to collapse on you when you get strong, combat this with running away, if they chase you they will die to Poison Trail and your allies, if they don't you turn around and attack them anyway. Even if you do die your Poison Trail will have helped your team immensely and you will have done your role as a tank anyway.