I'm not one for long guides or explanations, partly because I have no idea how to make a pretty looking guide, so I'll keep it brief.
This build is not the most effective, best support in the world, but it's pretty darn good and annoying. You'll probably never get ganked and have ridiculous damage thanks to your DOT, but don't forget your role.
You are a support. Don't steal kills, don't steal minions, keep it nice and steady. Ward often, whether it be your mushrooms or your actually wards.
Remember to utilize your invisibility in bushes (you can move while in them; pretty broken for poking).
Don't place your mushrooms randomly. Although you should never be carrying 3 mushrooms at once, analyze their team. If they always, always stick together late game and are only pushing down mid, before the next team fight, shove all your mushrooms to mid bushes and the side of mid lane.
Most importantly, have fun. Teemo's pretty darn good with that.

tl;dr: Pew pewp pewppew pew BLINDDD