Support Ahri is not very common, but I saw on her some good things to make her a support. There are some explanations about the items I've chosen to this build on the notes, so remember taking a look at them if you want to understand better my ideas.

More information about this can be found at Chapter 2 - Pros / Cons.

P.S: I am not a native English Speaker, so maybe my writing isn't very good, my apologies for that. There is also the fact that this is my first guide here, I've just created this account, I'm not very experient, but I hope the ones who read this can understand the main idea and evaluate it. :) Good game for everyone :) .

Pros
- On early game, the other team's AD Carry doesn't have much magic resistance, and Ahri is an AP based character, so, a nice damage could be dealt by her, and then her ADC partner could get the kill with the final hits.
- Nice range to hit enemies without risking herself (her Orb of Deception is a good choice for this, her basic attack also has a good range, but low damage - if you want to use her basic attack, maybe you could build some attack speed *).
- Good help on team fights (because of her ultimate skill and all the others), which can lead to many assistances and maybe even kills, depending on player's preference.

Cons
- No healing skills to save/help the AD Carry or other team members when necessary (she would have to use the healing spell, which isn't always available because of its high cooldown time).
- Her Orb of Deception might steal some farm when used, but not much.
- Problems with mana on early game, so you must be careful and use it well, make the Orb worth it with a good aim, fox! (With some items and levels this problem can be solved).

*If you want to build attack speed for her to use her basic attacks (fast pokes with no mana cost), I recommend you to use Statikk Shiv because of its passive (more magic damage, yay!).

What you could do here is almost like a normal mid-lane Ahri, but with some changes.

Instead of her usual boots ( Sorcerer's Shoes), use Mobility Boots with the enchantment Captain (this way you can increase your partners' speed) or Alacrity , so that you have lots, lots of move speed.

You should also add a support item to your build, such as Spellthief's Edge. This item will make your hits give you gold, so it's good if you are willing to poke constantly. There is also the fact that it gives you gold per time, and you can turn it into Eye of the Watchers, so that you ward well. Eye of the Watchers also gives mana recovery, life and some ability power, important things to Ahri (especially the mana recovery, since she spends a lot of it).

Another option would be Ancient Coin, but still I particularly prefer Eye of the Watchers.

Well, now let's talk about the AP items, which are some of the recommended to a mid-lane Ahri. Since you are going on bot-lane as support, it's fair that you have some armor to counter a little the enemy ADC's physical damage. That's why a good item is Zhonya's Hourglass, for it gives you armor and ability power.

But what if you meet the enemy team's mage on a team fight? That's why some magic resist is also important. So I've chosen Abyssal Mask, which gives me Ability Power (very important), the magic resist and the passive effect: decrease nearby enemies' magic resist.

I appreciate Luden's Tempest because of its effect (the charges with magic damage) and the ability power it gives to Ahri.

Other nice items are: Rabadon's Deathcap (ability power), Rylai's Crystal Scepter (life, ability power, the passive - decrease enemy's move speed), Athene's Unholy Grail (ability power, cooldown reduction, mana regen), Morellonomicon (ability power, cooldown reduction, mana regen, the passive).

Of those, I think Morellonomicon and Athene's Unholy Grail would be nice because of the cooldown reduction, using them both could be a good idea (many support champions, such as Sona, have lots of cooldown reduction items on their recommended set), but maybe you should focus on ability power and build Rabadon's Deathcap and/or Rylai's Crystal Scepter, it depends on your preference.

Well, in my opinion, there are three choices:

Heal - Use this if, since you don't have a healing skill, you want to heal your ADC some times. It won't always be avaiable, but it's better than nothing, and in some critical situations you can end up saving someone's life in your team.

Exhaust - Most supports use this, so if you want to slow the oponent to beat it to death with your ADC, it's a good choice.

Ignite - That would stop healing effects on the target, and would garantee you a kill if casted at the correct time, or just help with the damage.

I prefer using Heal, it's important to save the ADC or anyone else (even if only sometimes), this is one of the main functions of a support in my opinion - healing.

Early Game
- Poke constantly with Orb of Deception.
- Use Charm when possible.
- If you are close enough to the enemy, use Fox-Fire, but with caution, don't chase, it's just to deal a little damage.
- If you run out of mana, use her basic attacks to poke while you recover some of it.

Team fights
- Charm can be used to save your allies. When you see someone from your team fleeing from an enemy (there is a high chance that the enemy is walking on a straight line, easy to hit, them), you can charm the enemy so that your ally escapes.
- Use your ultimate skill Spirit Rush and hit the more enemies you can to help with the damage or get fed.
- Poke with Orb of Deception and even Fox-Fire, you are on team fight, you can be close enough.
- Spirit Rush can be used for fleeing.
- Remember your Heal spell, if you see someone with low life, use it.
- Rylai's Crystal Scepter and Luden's Tempest would be good items to have in this situation (move speed decrease + magic charges).

Wards
- If you build Eye of the Watchers, you can ward four times, so it's a good idea to ward the Dragon, the Baron or nearby bushes.

A support Ahri can't heal constantly, so it is very important that her AD Carry knows how to take care of himself - don't lose life too much, don't chase alone, play safe. It would also be good to have high range, so that the AD Carry doesn't need to take many risks, keeping a safe distance.

A good spell for the ADC could be Ignite. Usually ADCs use Heal or Barrier, but Ignite deals continuous damage, stops healing effects and would assure no escape to the enemy if cast at the right moment. So maybe it's a good choice (I've tested it with a Teemo ADC, it worked well).