I'am german, so sorry for my english :D
This is how i am playing Ahri. The masteries should give you the nessesairy couldown reduce and some more experience points. Then you can use your ULT before your oponent is able to.This is a big advantage and should save a quick kill ;D
This AP build also takes a very good amount of ability damage at every stage of the game. You have to try to be at least same as high as the mid - champion , or better one or two levels highter, to have the best damage output.
Try to gank other lanes after you are about lvl 6 (~lv 8) and bought some items.
The summoner spell teleport is for a fast return to your lane or a quick gank. Its very important in my opinion. I like it :D
The summoner spell clarity is for early game. So you have a little mana reserve to escape or to manage a quick kill. It isn't as important as teleport, but this way you haven't to save all your mana the whole time. In late - game it isn't important, because you haven't to use so much skills (i kill the most oponents very fast, if a gank succeded^^ So it wastes very less mana).

The only thing to say is that you have to keep a look on your HP. You have very less defensiv and too less hp^^ If you are gankes and can't escape you are instantly dead. So if you wish take flash instead of clarity or teleport :)

Mark: You "only" need a little about 12.000 Gold to buy all items. If an game hasn't end this late, then sell the mask and buy the hourglas.