Last hit Minions the most you can in lane, don't over commit to things and remember the only active that you have in this build - Zhonya's Hourglass can save your life more often than not
Manage your Mana don't use skills unless you know you will hit.
Buy Void Staff if MR becomes a problem and don't forget to place some Vision Wards and Stealth Wards

Hey guys, I'm Chrystalian and I'm nothing special, really just your average silver player. However I have been around the block a little bit and seen my fair share of Ahri play. I wish to share my most successful build (overall) with you.
Leave your questions, comments and concerns and help me improve this build!
Pros and Cons to Ahri

PROS
+ Has true damage
+ Has sustain
+High Mobility and Kill potential
+Pretty good lane phase

CONS
-Getting behind can be rough
-Skill shot reliant
- Blue Buff reliant
-Hard to play effectively

Secure the jungle for your jungler, don't forget warding totem and if invading get charm - wait till the last minute to take your spell if you feel that charm will be really bad lvl 1.
If you invade - help but don't stay too long as an extended invade will lead to you losing 5 - 10 Minions. As it stands an invade will already make you lose a few minions, try to minimize how many you lose.

At around level 4 if you have been avoiding getting harassed well you can trade however, don't trade unless you hit charm. keep harassing with Orb of Deception and remember to last hit those Minions! If all goes well you can easily get your Sorcerer's Shoes and be on with the game :D
Remember that in the event of you missing a Charm, they will try to take advantage of you not having it. Perhaps it was a bait and their jungler is waiting or you might take 200 free damage if you aren't careful. Take a defensive stance when missing Charm
Remember that with no Mana Regeneration items you are handicapped, you can only throw out at most 5/6 spells (at most) before being completely out of mana, be careful about when you use it.

Eventually (preferably) you will have to roam a bit, bot may be losing or maybe top needs some help. Roaming Phase isn't only the time to win lane. Ganking is something that Ahri and a bunch of other assasins are good at. If you have an even lane then push up and tell your jungler that you will be ganking bot/top. Note that when you kill bot lane that's pretty much a free Dragon they are two men down and if top is doing well then their top laner should still be up there.These were some locations you could dash to with Ahri's ultimate Spirit Rush This is the old map so, don't hold me to it if you can't ult over a wall or two anymore....Putting out Wards is important, vision control is half the battle and buying a few wards never hurt anybody. Just know where to put them.

Okay, Now is the time to co-ordinate with your team to snag those objectives, Ward up for Dragon/ Baron Nashor when the timer is has 2 minutes left, wards have a 3 minute life span.
ask yourself some questions like "has bot lane done well?", "have my roams been effective?"
Obviously you are in a dire state, like if all three lanes lost so play accordingly, don't try to 3v5 or anything of the like. When pressuring towers keep Stealth Wards and Vision Wards and keep the team focused on the prize. If your team isn't doing anything then that's a problem! Look at those timers and get ready for the next objective, you may need to teamfight before or during taking an objective. Take the time to think about who has more CC and AOE damage, these are really huge factors in teamfights. try to know your team inside and out and also, you should try to predict where the enemies are, "we have 3 people bot and two people mid, they might try to force a fight bot lane if they push too far out." this might seem simple to you but to others it may not be, knowing this is the difference between a won fight, a lost fight and a fight that never even happened.

When the teamfights break out you want to have :

1.

Zhonya's Hourglass
2. Rabadon's Deathcap
3. Sorcerer's Shoes
4. Spirit Rush
5. Mana

Note that you may only have magic pen + Zhonya's and thats OK.

When a teamfight breaks out your job as an assasin is to find the ADC and try to eliminate them, however if you can't get a Charm off on one of the carries then try to peel for your other carry, getting the tanks lower may be your only option, though with your mobility it seldom happens.
In a good situation you will have Charmed the Enemy ADC and have won the teamfight off of that, remember to have good focus on the carries and Don't forget that you have zhonias. The few extra seconds might be enough for your flash, an full combo or it might be enough for your team to save you.

At this point you have become a mage, you don't want to risk dying. Play it safe so that you can maximize your damage, and if you can still one-shot the enemy carries then good for you, but remember late game isn't solo; don't tunnel on the enemy carries especially if the tanks are near you, the Crowd Control can cost you your life. By this point consider if Guardian Angel is really worth it, you may need to sell it for banshees or some other item. Guardian Angel is huge to have at 30 minutes but not so huge to have at 60 minutes however it can still win you fights so be careful - if you are going to replace it be certain that this is the right choice.

if you are going to build Morellonomicon then do so early, maybe even rush it if you feel comfortable with that. Rylai's Crystal Scepter isn't the greatest item on Ahri but the slow has a few defensive and offensive applications, it can almost assure you hit your Q on the way in and back while at the same time being able to discourage people chasing you. Don't use rylais if you just want to maximize burst, use it in longer fights. Liandry's Torment is a questionable buy but if health stacking becomes an issue consider it as an option. Void Staff if they get Aegis aura and banshees or if something like that becomes a problem. If you aren't going to have Guardian Angel by the time mid-game teamfights start then it might not be effective so think twice about building it. If the enemy team is a health stacking team double Morellonomicon will be pretty good for east max Cool Down Reduction, with Liandry's Torment would be a decent choice against a team that stacks a lot of HP. Sorcerer's Shoes - Homeguard only really used as a first item if you are being pressured hard (think Heimerdinger)
Crystalline Flask isn't a terrible item against people who can out sustain you in lane. Consider this item if you think you will need more than 10 Health Potions or 5 Health Potions and 5 Mana Potions 70*5 = 350 gold, 5 more gold than Crystalline Flask. EnergyChampions like Zed will probably try to poke and push you out of lane. Health Potions are also if you can't sustain lane.

As seen above the Rune page is mostly Magic Penetration which is aimed to give me an advantage at squishy targets, when ganking other lanes with 29 magic penetration (with Sorcerer's Shoes) that burns through much of their magic resist during lane phase (usually 30-40) meaning that you have a significant damage increase and this works pretty well against MR items as well with Void Staff say they have 55 MR in total 55 * 0.035 + 29 = 48.25 Meaning that 48 of that Magic Resist is useless. Though unless they buy a magic resist item you probably shouldn't buy Void Staff