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.
Ghost - to chase & to escape, mostly to escape
Exhaust - I don't really use this for killing, mostly for defense or if I get in a 1on1 fight, Blind + Exhaust is a good amount of time for them to be disabled, so you can figure out if you need to run or if you can finish them off
Overall utility runes, magic pen for your poison and mushrooms, atk speed, cooldown reduction which I focus on with this build, and health, an extra 100 hps at the beginning of the game with a doran's shield for another 120 hps makes you pretty beefy
Utility for the Cooldown reduction, then up the Offense tree for more Magic Pen
Doran's Shield - I like it, gives you a lot of staying power so you can get to lvl 6 as quick as possible, your Toxic Shot is plenty good for harassing without using any mana at all
Boots of Swiftness - More movement speed all the time, if Move Quick is disabled you are still probably faster than the person chasing you, great for team fights, you can dart in and out, hit and run constantly - SUPER ANNOYING
Nashorâ��s Tooth â�� this is where the build starts getting good, atk speed, ability power, mana regen and cooldown reduction, everything you need. Start dropping mushrooms even more, drop them everywhere.
Zeal â�� I usually pick up a Zeal next, more atk speed and gives you a little more direct offensive power and faster move speed
Deathfire Grasp â�� Ability power, mana regen, and more cooldown reduction. This will get your mushroom down to a 10 sec cast time, continue to drop them everywhere. Also has a nice direct spell to hit the carry with in team fights
Trinity Force â�� more direct offensive power, move speed, snare, etcâ�¦lots of stats you can use
Zhonyaâ��s Ring â�� Ability power, adds dmg to your mushrooms, toxic shot, and blind shot. Nice active you can pop in team fights if you get focused
Void Staff â�� Ability power and spell penetration
Overall this Teemo build is a utility build. You have a very low cooldown and you cover the map with mushrooms, map control. You know where the other team is and your team is less likely to get ganked. You lose a team fight, drag the other team through some mushrooms. After a while you will really start to wear out the other team, they will be scared to even enter a bush or leave the main lanes thinking that a mushroom will pop on them.
Keep in mind this is a support\utility build, you will not be owning people left and right. You support your team and keep the other team in the lanes where you can see them.
Feel free to mid or take a lane with a partner, Teemo should do fine. Once you get mushrooms start dropping them everywhere.
Questions, comments, let me know how this works for you.