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.
Teemo makes a great support, because his Mushrooms are so versatile and useful in a support role, also blind is sufficient skill to counter ADC all game. This guide is done EXACTLY as listed, I tested several other builds, and most of them fall behind this one. Please follow it exactly for best results.
Early Game/Lane phase.
Your early game, is basically to hide in bush, let ADC farm, and when possible attack/Q the enemy if they get near or allow you to hit them. Also, if the enemy initiate on your ADC, pop out of cloak and blind the ADC so he survives. Most of the time you will be doing nothing, if they place a ward in the bush, sweep to kill it. You will be leeching gold + experience, which is critical since Teemo is fairly week before 6. So, you just want to make it to 6 as your main goal here. Your ADC should farm very well, because the enemy is scared to get in range to attack him since they know you will punish them. However, also the enemy should be farming well unfortunately in some scenarios. Just focus on hitting 6.
Do your first back once you hit 500g to get Nomads since this will boost your gold and health regen. Do your next back sometime after you hit 6, because you want to generate mushrooms and want to rush lvl 6 as fast as possible. Pick up your sightstone here. Key to Teemo support, is keeping up in levels/level advantage really lets you be effective. If you fall behind early the game will really be bad. So, essentially you will be in lane either hiding or mushrooming, just gainging levels/ambient gold for early part and playing defensive.
Post lvl 6 - Lvl 11.
Once you hit 6, then you can start generating mushrooms. This is where you become awesome. Put mushrooms defensively on entrances to your lane, and also in the side bushed for escape routes. Get your sightstone after 6, once you have this coupled with mushrooms, you should never lose to a gank since you will always have the advanatge. This lets you team easily get Dragon / bot tower also, since you have positioining advantage.
Your next goal, after hitting 6, is to get Dragon and hit 11. Yes, its kind of boring, but you need to get 11, since this is a key level that your mushrooms generate faster, and you have higher range on placement. Once you hit 11, you are "good to go" for the rest of the game, since this skill set is sufficient to be effective. So, stay in your lane until you hit 11, and you will have good gold and experience to be effective for the rest of the game. This part takes patience, it may take around 20 minutes to accomplish this, however if you do it then you should be set for a win.
Get Dragon all the time possible. Kill the river scuttle, because it breaks your river mushrooms. Then you can mass mushrooms around Dragon, and it makes it very hard for the enemy to attack you. This same strategy works later for Baron also.
Post lvl 11 / end game
Once you reach here, you will be reaching full CDR maximum (40% or 45% depending on your mastery). So your goal is to place mushrooms in all the bushes, and future objective points. Also, you can split push like crazy, since you can setup vision perimeter and never get ganked successfully due to your cloak and escape skills.
Follow the item build exactly, once you are max 6 slots and have enough gold, sell Mejai's and buy Deathcap (Deathcap is better than Mejai). Once you have enough gold, next sell Sightstone/Oasis and buy Voidstaff. This will give you mega damage, and just make sure to keep your mushrooms up, since you will lose the ability to ward and your mushrooms need to take that place.