I tend to go 12/18/0 on my masteries because although danage is good, the 12 slots in ferocity give me more ap and more spell vamp. This combination will make you powerful as an AP Teemo. Also, as always, it is good to have thunderlord's decree.
For runes, I focus on getting as much ap as I can and as much armor and magic penetration. I also like getting a bit of extra health because Teemo is a squishy champion.
I don't have any images for this but my upgrade path is: E,Q,E,W,E,R,E,E,Q,Q,R,Q,Q,W,W,W,R,W
I build the items one by one, in the same order as they will be mentionned: Liandry's Torment, Sorcerer's Shoes with the Furor enchantement, Nashor's Tooth, Lich bane, Rabadon's Deathcap and Luden's Echo.
Try tjis out yourself, I won't have to explain why this is good sonce you'll see it. If you still don't succeed as an AP Teemo, all I can tell you is practice.
AlphaDoritos is out!