Recently with the killing of teemo as an AD champion carry, I've found that for a few patches he became something more akin to an AP burst champion.
However! with the addition and reworks of several items such as wits end and nashor's tooth he has become something more along the lines of a reliable damage sustained AP burst, whom can walk toe to toe with most conventional mids and in a lot of cases shut down completely a lot of the tankier top lanes. and I will use this guide to explain how.

With the way teemo has been changed and made to be something far stronger than what he used to be, he isn't confined to a single specific lane, he can easily swap top and mid at any time with nearly any champion I have been finding as of late through playing ranked matches. Starting out with a doran's ring and two potions in either top or bottom works quite well.

You will quickly enough find through common teemo tactics of dodging and weaving, pot shots with your blind as people move in to last hit as you will almost always have a melee champion top will result in them backing off, or missing the last hit's due to a blinded strike. With early game dominance the main thing you usually must be afraid of is a jungler due to a double slow or stun is what tends to murder you pre 6 when your lane will be covered in mushrooms. Depending on how the lane goes, you will often times finish sorcerers boots first and move onto another two doran's rings afterwards (I know what you are thinking, why sacrifice getting a good item for placeholders?) But in all reality, he needs the mana regen along with the AP, the masteries which I will explain next will tell why.

Use the bushes to your advantage, just be careful to not overuse them, plant mushrooms in places you see the opponents weaving to, make sure creeps avoid hitting them by leading them away and they won't suspect it's mined until they hit the mushroom, a blind,poison dart, ignite and potentially their doom if not have to go back due to being nearly dead further ensuring your lane dominance.

Much like how you play top but with a couple of rudimentary changes, most mid's will either try to last hit with skills to keep themselves out of your range, and in that short time period where they commit to getting a creep kill, and you come to learn how they effectively do it by observation, you will be able to land a quick blind and if you are sure you won't get hit or get a better trade off a poison hit in as well before they can even strike you, they will move off with you having the better positioning to better harass.

Most ap carries are to used to an unaggressive champion mid, they tend to dance around in and out and it will give you many opportunities to land a hit, if you can keep your eye on the mini map, wait for ap nukers to waste their abilities that would otherwise harm you to get a last hit, rush in with move quick pot shot and retreat with no repercussion. Do much the same as you would top, place mushrooms where they weave, or where they might try to dodge, push them into it and often times you will find they were trying to get a gank from a jungler to help them and die before the jungler can even intercede.

I have found through a few different attempts that a 21/6/3 mastery set up works the best. All the ability power additions down through offense, the utility tree for the initial mana regen, and the defensive tree for scaling health per level and depending on which lane top or mid armor, or magic resist for your last two points.

The five percent damage boost from your ap both early and late will start to shred fairly badly along with the intended item build, amongst other things with all the rest of your scaling ap per level.

Using the magic penetration reds, along with scaling ap per level runes for yellows and blues has worked wonders along with the flat ap quints I run, the reason for that being with the flat damage your initial hits will hurt more, but you will still scale into late game without hurting your overall presence due to sacrificing ap for mana regen runes which is why I go for doran's rings to deflate the issue early and sell the unnessacary items later.

To be honest, the build you do won't change very much, it's more about what you build in what order to navigate damage against people, by the time you have gotten sorcerers boots, two to three dorans rings, built your haunting guise most champions have a hard time fighting you due to the harsh magic penetration you have, making it worse as soon as you add in wits end, the cooldown and ap from nashor's tooth along with the 15%(making it roughly 20% a hit of your ap due to talents. Lich bane in or sheen as soon as possible afterwards and I kid you not...
Due to multipliers...
Auto hit (99 ad) +
Toxic shot (50+50-70 damage) plus poison tick starting for another 60 or so damage at this point = 279 damage +
wits end (42 plus magic resist shred/steal)= 321 +
5% ap on hit from talents+ 15% ap on hit with nashor's tooth(nashor's usually 15+45 and 5% adding in 15 damage more for 75 total plus 321 = 396 autohit damage
Blind shot doing an ap nukes damage of 500-600 damage, lich bane adding in 50+270 damage=1000 damage burst with 400 damage auto attacks and mushroom dots in with liandry's burn. You can effectively burst most people down quickly with this sudden damage that they do not even expect, tanks, ad carries, tanky tops, junglers, supports. The only down fall you face is that you are squishy but your mobility usually more than makes up for it with move quick. I hope you enjoy it as much as I do! Have fun out there!