This teemo build stacks magic pen from runes, this is central to the entire build and playstyle. it is what helps teemo destroy tanks and carries alike.

Magic pen is so important because of teemo's E skill, toxic shot, which does magic damage on-hit and over time. As well as Mpen being profitable on every mushroom and blind, its also applied to every auto attack AND every tick of poison, which makes it an exceptionally efficient stat on teemo, and very profitable in killing tanks. Every champion has a base MR of 30 and we acquire 29 mpen in this build. We do it without using mpen boots so we can have the atk speed boots instead, and without a void staff so we can build tankier, to really make use of this synergy.

Wherever you start the game as teemo, your job once the first couple of towers are down is the same - create an overwhelming presence of mushrooms to give your team incredible vision, set up ganks and securely press lanes.

Beggining:

In laning phase CS and grab a kill or two. Early game is difficult. Don't fret if your are slightly feeding after 10-15 minutes. You WILL get kills and you will really start to steamroll towards the end of the game. Typically i might have a couple more deaths than kills after 10 or 15 minutes but if you stick to the gameplan and be patient it will come good.


Early Game:

When a couple of turrets are down, try and build up a presence of shrooms around the central areas of the map. Concentrate on this and try and be extra cautious during this time as dying delays your shroom domination.

Mid Game:
Profit from your shrooms with your team.

Late Game:
Maintain shrooms and build those tanky items for the teamfights. AD Carries are always the primary and easiest target.

Teemo's passive is awesome. Be sure to make use of it creatively. You can use it in lane any time you will be out of vision for a few seconds. You can use it to intercept enemies with the help of a Mushroom. It bumps your attack speed which makes good use of all your ap and mp.

Don't create "shroom hell", patches. It is fun, but your shrooms are too important for vision. Build your shrooms up around the entire mid lane and surrounding jungle. Over time the entire map becomes a "shroom hell", this way.

You can lay shrooms designed to be ran into by enemies, but also ones designed NOT to be hit. It can be a great idea as sometimes your shrooms in an area can all be run over by one tank who can easily regenerate before anyone from your team can get to him, so you lose your vision and map control for very little. If you place the occassional shroom designed to AVOID being hit then you can maintain excellent map vision without the risk of losing everything without return because of one roaming tank.

Here are some example of where you might place shrooms to be hit (in red) vs to survive and give long term vision (in yellow)
Be efficient with your shrooms. Try and make sure you never have 3 in stock. If you notice yourself with 3, lay one in the nearest position which may be remotely useful. Again, over time the extra shrooms will ready add up. Likewise if you are about to go back to base you should look and see if you are going to get that third shroom within the next 5-10 seconds, if you are its better to wait and lay it before going back.

Support:
By the end of early game you can often have more kills from support than any other lane. Allow your ADC all the creeps, but take those which he wont get and go in for kills with just as much intent as if you were mid or top. Move Quick and Blinding Dart are typically most benefial to level first.

Mid:
Its always a difficult lane but allows you to build up a shroom presence in central areas very early. Dodge the aoe and skill shots with move quick, ignore blind, shroom dragon and centrally.

Top:
Its usually best to level toxic shot, move quick and blind equally. Going top can sometimes delay your good midgame.

Which abilities you choose first should vary against who you play.
Teemo is very flexible. He can play top, mid or support. Against mid, Blinding Dart is very useless. As support, it is your primary tool. As top, Toxic Shot is always useful, whereas its not nearly as important when supporting. Against most mids, it is best to level Move Quick early to dodge those AOEs. Whichever role you are playing you should assess which skill will help you most, rather than stick to a set pattern.

Potions:
Make sure you have a stock of hp/mana potions throughout the game. I like to never leave base without 2 of each. Mana potions are especially critical as they can allow you to participate in a team fight when you otherwise couldnt.

Boots:
AS boots are better than Mpen in this build in nearly all circumstances. This is because we already take alot of mpen and more, unless the enemy is seriously stacking mres, wont be necessary.
AS boots are very useful because it helps you deliver the damage from the mpen and AP that you have stacked/will stack, on every auto attack.

Must Buy Items:

Nashors Tooth-
Integral to the whole build as it lets you use your ap in auto attacks.

Rabadons Deathcap-
Absolutely necessary as your primary source of AP

Liandrys Torment-
Necessary for the magic pen which is critical for the build, AP and HP.


Generally Good Finishing items:

Banshees Veil:
Health, MR and a spell shield increases the chance of you being able to get in and out alive. Teemo has great innate defense against auto attack champions with blind, so this item balances him against ability orientated champs too.

Guardian Angel:
Gives you that extra bit of security to get stuck in during late game. Due to map vision this item should only be procced during a team fight, which makes it extra beneficial, it also gives time for Blind to come off CD, which gives you a massive chance of either making a kill or escaping.

Other items:
Lichbane - I dont feel it synergises great with teemo.

Deathfire Grasp - Good for assassin teemo

Zhonyas Hourglass - After your core build the extra AP isnt really necessary and i feel tankier items are more beneficial, especially as you can deal so much damage to tanks you need to be able to take some.

Randuims Omen - Can be a decent choice against heavy AD comps, take instead of Banshees Veil. Slow can be useful offensively and defensively.

Hextech Gunblade - No synergy at all with this item. The AD is pointless, the spell vamp is useless.

Frozen Mallet - Tanky item similar in total effect to Randuins omen. The ad is useless. There are better items for any situation.

Void Staff - Its better to get the Mpen from runes and Liandris - for its passive and HP

Wits End - This item does have amazing synergy with the core element of our build. It really boosts your auto attack damage. However, I feel it is better to get mpen from runes and liandris which allows you to be more tanky and makes use of the liandris passive. The extra damage is nice but not needed, and getting mpen as a flat stat rather than an increasing amount on hit allows us to do our max damage instantly and makes our mushrooms and blind far superior.

I am no pro player, they will say to take more armour. But i find the mana regen/level seals quite critical as we buy no manaregen items and we need to make sure we can always pop a shroom out when one is available. Magic pen is critical for teemo - explained in the basic gameplan.

i think the defense tree is bad for teemo in almost any situation. manaregen, move speed and summoner spell cooldown reduction are all things which can help teemo avoid a situation where he would need a little mr or armour. When taking damage become unavoidable in lategame, we already buy tanky items.