Outdated, and, quite frankly, there are a lot of ways to improve this.

Nice

ap tee on duty

Jup

Nice guide, like it pretty much. But im fully against nashors tooth. Really no need for that att speed, yet you pay for it if you buy it. Becuase you hit once and q him and run, so no need for att speed.

Better turn that fiendish codex into morellos evil tome for ap and cdr for little gold. All your other items are great.

I would ad a zhonas, for the active effect and ap and armor ( litle more durability)
Maybe ad Rod of Ages after Rylais, if you want even more health.
Abyssal staf great because of that mr you get and ap.
Deathfire grasp also nice:
Morellos ( 20% cdr) plus deathfire grasp ( 15%cdr) and4% from masteries is 39% in total= excelent!
Also, that active is good bonus to teemos damage output, especially if he gets in a 1v1 fight. And kages lucky pick gives you extra gold on start.
So nice guide, just more optional items.

And warding, put more positions for shrooms. Those you mentioned are for early game. there are more spots if your team is pushed or if your team pushed your oposing team.

nice build with a lot mushroom dmg

I play Teemo AP, and this guide are good.

Well i think AP per level will be better and i'm sure you can use Magic pen on Quints too .
Also in Items as overall items you have 5 . And there are 6 slots :P Still there is need of optional items too . Good 6'th item for Ep teemo might be lichbane :)
Still overall the guide is helpful , might not be perfect , but can teach most of the teemos the basics of mushroom planting and really can make them see full ap as an option .

I don't usually think of Rammus as someone who stacks MR early game.

Good guide. I like the shortness, because I can't be bothered reading walls of text.

There are some grammar mistakes in your guide.

Also, the mushroom map, please extend it. Place more spots, on both purple's and blue's perspective, instead of only 1.