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 sort of a well rounded build for teemo. It has elements of DPS and AP. I personally use this.
Life steal early in the game allows you to lane longer. Attack speed and cooldown reduction runes compliment either a DPS or AP mood, which ever you feel is needed at the time.
The skill order is more of what you think is best at the time. I like to start with Toxic shot first though (It can be used to farm minions well and it punishes champions that retreat early in the game).
The items listed are the ideal final items, buy others and eventually updrade. Buy life potions as see fit.
Rod of Ages is optional if you want to strengthen your mushrooms towards the end of the battle to lean towards an AP build, if not, buy Last Whisper instead.