this is my first build, so it is very simple, don`t be too rough on me ok?
well starting now: the only thing you need to know is that you have to always save one ult charge to flee from battles, or you will feed. and please go mid, don`t try any other lane with her, she is very squish so against 2 heroes is badd idea and u get your ult slower and her ult is OP, and needed for her survivability. and of course don`t try to engage more than one enemy in the front line let this to the tanks, you stay behind, hit charm and get a kill with your skill combo!

PROS:
-good damage if you have the correct items (you should aim to have them)
-great range on the skills, so you can keep a safe distance and still kill.
-have true damage that stacks with AP (need to explain anymore?)
-triple blink, so you only die if the disable you, make sure they don`t(need anything else?)

cons:
-dies stupidly fast, really is that fast
-if ult is on CD, lowers survivability a LOT, so don`t go rampaging without looking at ult
-have to hit the skills, in the right order, to have an effectiveness on fights (so don't be blind, and use your fingers!)
-totaly item dependent (really don't try anything if you don't have enough item)

get mid, and start harassing on level 3, use charm-orb of deception-foxfire combo to get a great damage, keep using only the orb is a waste of mana, because is easy to escape, so try to use it only on creeps to heal (with your passive) some times, if the enemy make it possible, get close enough to hit the fox fire without the other skills to give some damage, i know that can be impossible some times and you will need to use only the orb of deception to keep a safe distance, to hit the orb of deception easily try using it when the enemy comes to harass you or last hit a creep (first option is easier). and as soon as you get your ult is just go killing like crazy, it is a great thing to go banking on lanes at level 8, because your damage will be already great, and use your ult to get close to hit the skills and give no possibility of escape, your will be very useful to last hitting heroes if your other skills are on CD. and most important don't think twice to use your ult if running away from some one, it`s rage is very good so it can pass by almost all walls in game.

in my item biuld the most important thing is to get first the simple items rather then go making expensive items, so get chalice and codex asap (but don`t forget the boots! they`re what make you awesome, and will be much easier to hit your combo), that will give some mana regen to keep you from leaving lane by loss of mana and if your not going that good, try get Hextech revolver before completing rod of ages(to get a little more damage and survivability) , then go rod of ages, and then finish will, that will give you something to make you stay out on the game for some more time. If you are needing an extra CC (rylai) you can make it right after the rod of ages, and then make the other 2. rabadon`s is the last one because is too expensive and don't give you any survivability (what you need much with her) but if u get enough feed it will come pretty fast, and then it is just ownage. But in some cases when you don`t need so much cc you can get Rabbadon before Rylai, so to be stronger.

normaly, people say to maximize the orb, but i realized that is not the best option, it is better to get fox fire first and then go level more orbs, because fox is better to deal damage in 1x1 (mid is always 1x1), charm just level 1 at early because the disable don't get higher enough to waste points. ult always when it is possible to level, of course it will be her main skill, so use it with wisdom.

i rather use heal to survival than flash, flash makes u run and u lose a kill, but heal got a great survivability on early lane, and normally make you get kills you otherwise wouldn't. and above all else she don`t need flash she have triple dash!!!
i use ignite, because is a great way to amplify your early damage, and to reduce healing on late, like WW, soraka, or anyone like that. believe me it is very annoying when you use a full combo and then, as you have no skills the enemy escapes with 100 life and heal himself so you can not kill him.

masteries to get some mana regen to help you on early game (harass some times takes a LOT of mana) and mostly Ability power improvement masteries, she is a main damage character, based on AP so with more AP, more damage and then more useful you are.

some times you will need to use skills to save a friend from death, it is very easy, just hit charm so it will be slowed and will not attack, if there are more enemies try hit the charm in the one that have the better disable ( CC ) and use your other skills to scare the other ones, if you already have rilay`s staff you can try hit most enemies as possible. in team fights try stay out of any unnecessary danger, keep your distance using your skills (they have a good range so this should be no problem, if you know how to hit, and Swiftness makes harder for the enemies get you if you keep moving(seriously you don`t need to attack, her AD is useless)) and use your ult only to get a kill then get out of the way so you don't get killed, or if your team had engaged badly on the fight just run, RUN LIKE HELL.
At laning phase try to gank as soon as your ult is up, go to the lane, and initiate hitting your charm, DON`T fail, and is a double kill for you.

the best way to farm creeps with her is to wait until your creeps let the melee creeps with half of their lives, than you use the orb of deception, hitting both melee and ranged creeps, and then last hit ranged creeps on normal attacks, if you are going to take blue buff on neutral creeps, try to use all skills as soon as they are out of CD.

The last but not the least, the only way to get really good with her is to train, play a lot with her, to get used to the skills ranges, damages. and to be able to hit the skills on a great distance the only way is to train and train hard, start going some coop vs. AI, then go 5x5 to get the real Ahri on the job!
And a hit that i found very useful is to use Smartcast, it`s awesome, you can use your combo in a split second and confuse your enemies so they won`t know what hit them.