Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is a build centred in a guerrilla teemo and mushrooms (with a good AP amount). It has a lot of similarity with this build (the most famous AP build in this page at that moment):
This build in centred in Cooldown Reduction. You should raise the 40 CDR (the max cap) with 9 (masteries) + 5,85 (runes) + 25 (nashor tooth). That is 39,85 of CDR (You can use 1 glyph of Celerity and 8 glyph of Focus for a total CDR reduction). With 40 CDR you will generate a mushrooms every 13 seconds. You will be able to use binding dart every 4,8 seconds.
Teleport is very useful in that kind of teemo as you can teleport to you own mushrooms, and you could go to any part of the map if you spam correctly the mushrooms. Flash + Zonya Ring will help you to survive and dont lost you Mejai stacks.
1) Start with a Meki Pendant for mana regen. Rush to Nashor Tooth for maximum CDR at start.
2) This build is focused in mushrooms so get Boots of Mobility. Boots of Mobility + Teleport = Mushrooms in everywhere.
3) Now a SoulSteeler. You must be careful and don't die. With all the assists (and some kill of your mushrooms, you will have a lot of stacks.
4) Now is so late for a Rod of Ages, I recommend instead a Rylai Sceptre. It will give you some survivability, some mana (mushrooms spam is so mana dependent), a lot of ability and you mushrooms will have mayor slow effect. You can build instead a Rod of Ages if the match is going to be long, that will give you more mana.
5) Zonya ring will boost your AP and save your live sometimes with his active.
6) Finally Lich Bane for more mana, ability, magic resistance and Movement (combined with your Boots of Mobility you will be Flash out of combat). And of course, add a lot of damage to your attacks with your ability (combined with you poison (scaled with AP) you will do a lot of damage).
PD: A final note, I start maximizing toxic shot for better farming (remember to spread the poison).