Ahri belongs in the mid lane. She can easily poke and bring targets down to half health without them being able to touch her. If the enemy is playing too aggressive, use your E to pull them into the turret. You will become nearly impossible to kill in the late game due to the spell vamp and your ultimate. Ahri can easily 1v1 bruiser's and ADC's (Master Yi, Varus, Miss Fortune...) The only thing you have to do is have a safe early game.

This is the last game i played with Ahri, even having a bad early game where i fed 4 kills, i was able to turn it around

To get started you need to have a good farm. Make sure too last hit minions and don't push the lane. Even though if someone tries to gank you, you have your ultimate to save you, but you shouldnt try to waste your ultimate. Optimal farming would be directly out of the towers range so you arnt competing with the tower, but also that you cant be ganked. Always poke your enemy with your "Q" during the laning phase once you get your Tear of the Goddess because,

1. You will increase your maximum mana
2. Interrupting their CS farm is extremely beneficial and will put you in the lead

Just make sure to play your lane safe.

Ahri's passive can save you a majority of the time and also increase the amount of CS you get. Instead of recalling after getting a kill in mid lane, use your passive to regain your health. When using your passive, aim for the biggest group of minions with you Orb of Deception (Q). By the time you opponent gets back to mid lane, you will have regained a majority of your health back.

Ahri's Ultimate is great for cleaning up team fights. After the fight, you are able to hunt down any of the survivors and do tons of damage. Nobody can really escape unless you get CC'ed. Even after a CC, using your Charm (E) may get you the kill
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This is the first guide that i have made and is not done yet. Comment on things i should add.