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.
If I see another Teemo start out with an Amplifying Tome I will die inside.
You max out Move Quick before Blinding Shot so you can get in, do damage, and get out instead of dying instantly when they focus you. A 2 second blind is enough for early/mid game and please for the love of God stop harassing people with it. Get Boots of Speed and a dagger on your first recall. You should honestly be building the boots into Merc Treads, but on the rare occasion the enemy team has no AP/CC, I'd grab Boots of Mobility for getting places more quickly. Bloodrazor eats up most tanks and you'll have a lot of attack speed early on, which is important for Teemo since his poison applies every hit and gives you extra damage per attack. Frozen Mallet is a necessity for Teemo because you have a horrible health pool without it.
Teleporting to mushrooms for surprise kills and to towers to save them is efficient. Also use teleport late game to backdoor towers as you should be able to **** all over them by that point. Flash is extremely good for Teemo as it gets him out of almost anything since after the flash he can haul *** out of there. Your move quick will most likely be knocked off if you need to flash out of a fight, so you'd better be sure you won't just get flanked after the flash so you can run away for long enough to be able to reactivate it.
Flash is also good for getting more hits on a fleeing enemy. Teemo can most likely move quickly than almost any champion, and with a lot of attack speed he'll be getting a lot of attacks in late game after a flash forward. Please don't try to 1v1 someone unless it's early game and/or you've blinded them after they've hit a mushroom.
Mushrooms are good, especially heavily placed in a lane near the tower to prevent pushing (obviously away from the middle as a creep wave will just hit them). An oracles will just **** it up, though, so keep them in bushes most of the time and in thin paths. If someone gets oracles or a vision ward in your lane, STOP PLANTING MUSHROOMS. YOU ARE GIVING THEM GOLD. Only use them to stop creep waves when they aren't pushing. Stupid teams will never buy oracles which will make your job of preventing pushing a lot easier since they'll hit a ****load of mushrooms on the way to the tower if you're placing them well and you can just **** all over them as a team. You shouldn't count on the enemy team's stupidity, though.
Never rely on mushrooms too much because they're easily countered. This is why you don't build AP. Toxic Shot's poison does benefit from it, but not nearly enough to make it worth it. The extra damage from the poison, however, gives Teemo's DPS a healthy boost.
I've played probably over 200 games with Teemo and even with the changes made to him I still think that AP Teemo is extremely situational and easily shut down. You are more of a liability to your team than an asset if the enemy team uses vision wards/oracles or builds magic resist.