Hello, I am GameOveRus and I welcome you to my guide!
This guide is the very basics of Singed you have.

Runes are standart for AP-Offtank:
Magic Pen. Marks: for extra damage thoughout the game.
Armor Seals: Defense we need
Magic Resist Glyphs: Same as Seals - Defense (We are tanking, are we?)
Ability Power Quints: for some raw damage early.
You can also try Movement Speed Quints. (They work perfectly all the game, synergizing with Boots of Swiftness, Singed's Ultimate, making you run in gront of enemies applying your Poison all the time until they die).

We go 9-21-0 mastery set for AP-offtanks (Yeah, I am never tired of saying *Offtank* :D)
Nothing much to say.
Seriously.

A mix between Armor, Magic Resist, Mana, Ability Power and Health which is kinda awesome on Singed.
Special look I want to give to WotA - when you get this item, your poison will heal you, which is just NIIIIICE.
You get hit - your poison regenerates it back, and even more!

Ghost: for running around - synergizes with Boots of Swiftness, Ultimate. Makes you run so fast nobody can catch you.
Ignite: for early game First Bloods.
Decent choises:
Flash
Cleanse
Fortify
Teleport
Clarity

Ok, that's it.
This guide is made more for myself, 'cause I can sometimes forget, what i build :D
Credits go to:
Me, the Mighty GameOveRus (Ego OVER 9000)
EvilDice from MobaFire (His guide served to me well, until I invented the idea further and it resulted in this guide)
To all my friends - I love ya all :)
Thank you!
Read, comment below, subs-
Ah, there's no subscribe on MobaFire.

"How about a drink?" (c) Singed.