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.
with rune updates the mana yellows may not be the best but i will have to do more testing, an early meki pendant may also be very helpful
you can no longer buy health pots and doran's shield but i still think doran's shield is the best starting item
I need to do some experimenting with the runes, especially for the quints. However i feel fairly certain that the magic pen reds, mana per 5 per level yellows, and flat cd blues, are pretty solid. The movement speed quints are nice he is the swift scout after all. they may be the most beneficial for him late game however early flat health or possible ap or even attack speed would be nice. I will update this when i have the time and ip to try these other quints.
For masteries i just run a relatively basic caster set, nothing special. you could pick up extra buff duration but i don't see too many changes that should be made to these.
For items i start with a dorans shield and a health pot. the extra health is nice and the health regen also greatly improves his laning ability. a saphire crystal and two health pot start may also be possible, but against a champ that has any harass i don't see this being successful at all. then on you first trip back get basic boots. at this time you have to make a choice, if laning is not going well or the other team has alot of cc go ahead and start your catalyst. if its fine you can pick up nashor's tooth. the first three items you should get though are sorc boots nashors and banshees. then pick up a haunting guise, with this item your shrooms will be hitting for true damage on most champs which is amazing early. the extra health also gives you survivability that you desperately need as teemo. after these items i tend to move on to get ap items, any ap items are fine some i recommend are rabadons, hourglass, void staff, and possibly an abyssal scepter. guardians angel is also a possible item instead of banshee's if you are facing a lower cc mostly dps team. if you build this you may want to get chainmail early and hold off on finishing it til later. the biggest part of playing teemo is by building off of what you need. these are all good items to use but you must learn the order you personally need them in.
with teemo there are two ways to skill him, you can either max blind first or max poison first, poison is much more beneficial, however if you are laning a strictly auto attack champion you can max blind first. besides that always pick up r when you can and pick up a point in move quick at level four.
i recommend ghost and flash, you want to follow this on teemo as they help for great escapes and chases, the only other spell you should consider is changing out ghost for ignite.
as teemo you can solo mid and it is the best lane for him. it allows him to shroom the majority of the map very easily which you should be doing as teemo considering it is possibly his most useful ability. try to harass early and once you hit six shroom everything, first protect yourself on both sides then move on to the rivers. be careful no one is going to be in the river where you are going. and it can be best to save up three then go and put them all down when you know it is safe. as far as teamfights go you need to try to catch them in the jungle or bait them into it. this is where all your shrooms will be and your potential to keep your team safe while getting easy kills. you should not be afraid to flank the other team if you know you have an escape path of shrooms, teemo is great at finishing off champs that are trying to run.
Solo mid is ideal for teemo as long as he can handle the match up.
Against auto attack heavy champs you should level up your q first and wait for a good opportunity to blind and then follow up with auto attacks. Against champions like ashe i also max q first her main damage is volley but its easily avoidable and then you can follow it up with blind and harass. against harder lanes and mages you have to decide how great their lane control is. if it is a champ like annie that you should be able to control the lane against you should max your poison first. if its a lane that will be very tough you should max your blind first and just get in harass when you can.