When going mid, be carefull to every time keep an eye on the bushe. If you play well, the ennemy team will sometimes send one or two more champ on mid. That can be very dangerous if you have already used your E.

On mid, every time you kill a minions, just laucnh a Q spell / Auto attack on the ennemy player

Just farm and HARRASS until level 3 . But Link, how i must harrass ?
It's very easy, let's say everytime you hit a minion when farming, you laucnh your Q spell/an auto-attack on the ennemy player. He will get really tired of you, and he will back often. At level 3, you can try to kill him after harrassing him a lot.

You have some items. You MUST keep farming. Ahri is so freaking good to farm. When you buy a new items (who increase your AP) , minions will suddenly become very very weak. Just keep using your Q spell before ennemy minions encounter your lane minions, and then just last hit them.
When you have enough items , you can start the real "hellish" period.
When you are in a 1v1 :
-Launch your Z first, just to lower his HP without making him go back
-laucnh one or two Q spell/E spell at him, just to lower at the maximum his HP
-They nuclearise him with your Q/E/Z combo. If he have something like 200 hp left, kill him with your R, and don't forget to flame him, we never know if he have a health summoner's spell.

NUCLEARISE YOUR FOES :DDDDD
With Ahri, just launch the deadly E/A/Z combo when you have nearly all of your items, and watch your foes health bar lower at the speed of light. Really easy to score a Double/triple/Quadra kill if you're good :D But be REALLY carefull about other Ahri/Morgana, or any champ with a incapability spell. It's really dangerous, because EVERY player of the team hate you, and want you dead. So be carefull about htose, unleash hell, and have fun :D

~Linkdeous, playing with Ahri for now 3 years