Singed is one of my first tanks to use. I built him so tanky i found he wasnt doing much with such low cc. I thought he wound be more of a threat if he had more dps and speed. I built off his passive. With mana he turns it into more health. So i figured if you build only mana and throw in a acrhangels you can turn mana into a ton of health and ap. With ghost and insanity you can tower dive and catch anyone with cleanse and toss back to a teammate to finish. he is a great team fighter

Pros:
-Great at all stages of the game
-Easy farming
-Great kiter
-Tower dive master
Cons:
-Dont expect to carry or kill people fast

With the exception of vitality i focused on using runes on dominating early game. The insight gives poison and toss greater power. Fortitude gives you great tanking abilities so you can take the focus of the enemy while a more dps laning partner gets the kills. The intellect gives you the possiblity to spam poison longer for better farming and get more health with passive.
For better tanking abilities early on take resilience armor instead of vitality so minions do less damage on you while you harass the enemy.
Potency quintessence give better dps but make you less tanky.

With my masteries i went mainly tanky. I think masteries help more when it comes to tanking then doing damage. I went upgraded cleanse for better cd and ghost for cd and duration.

I stared with mana gem for more mana for harass and farming. Pick up a heath and mana potion for when each runs low. If you are in the lane long enough to afford catalyst get that first. but if you are forced back before get boots of speed. Get Rod of Ages as fast as possible then finish boots of swiftness. If the enemy team has a lot of stuns and slows get merc treads.
After RoA get another catalyst and if the other team has more ap finish banshee veil or if they have armor finish frozen heart first. If balanced get the catalyst then a glacial shroud.
Crystal Scepter is almost op in the hands of singed. His poison will slow anything in it. Plus you get the ap and health.
To finish it off i get archangels to get even more mana that turns into health. And with archangels passive mana turns into ap.

If you want to get a chance at first blood go toss first, if you wanna farm go poison trail.

Ghost is almost a must but if you wanna a better chance at first blood get exhaust over cleanse, but cleanse is better when chasing multiple targets and has a way shorter cd.