Ashe support has come and gone in the meta over the years, and while mainly played in the ADC role, Season 11, has brought her back to the support role.
From her humble origins as a Marksmen/Support she offers both damage and utility to her team

No other support benefits from Imperial mandate as much as Ashe, able to utilize it on every CD.

The changes to Wardstone means she can take advantage of every bonus it gives.

The items in this build all proc on her E, and with high CDR, it allows you to stand back, and spam Buffs and Debuffs during engagements.

At full build her ult will have less than a 20 second CD!

Sorcery
Arcane Comet: extra poke damage, scales well into late game
Mana Flow Band: one of her weak points is she is mana hungry early game, this allows you to stay in lane and poke a little longer
Transcendence: Extra CDR
Scorch: extra poke damage


Domination
Ghost Poro / Zombie Ward: your choice here, if you prefer to go sweeper, zombie, if you prefer wards, go ghost, both have pros and cons but equally as good, adds utility to your sup kit.
Ultimate Hunter: This is an amazing rune on Ashe, it gives a massive bump to your Crystal Arrows CDR, and allows you to spam it late game.

Lesser Bonuses
You want to go CDR, Adaptive, and either Armour or Magic resist based on your matchup.

+

Most of the time you want to take Flash and Exhaust

Exhaust is great for when you get jumped, and need to get away, or to exhaust the enemy ADC to take them out.


Another option is Cleanse, Exhaust usually wins out, but if you are against multiple hard CC champs like Lux Morg or Swain, Cleanse is the safe pick

Imperial Mandate
CDR: is by far the most important stat on an Ashe support and this has a decent amount
AP: her Ult has an AP scaling of 100%, so +40 is 40 extra damage, and each additional 15, is just extra umph to your soon to be spamable Ultimate
HP: Ashe is by nature a squishy support, so every little bit of survivability helps, against assassins.
Mana Regen: this will help you stay in lane longer during the early game, without going OOM.
Passive: abilities that slow do extra magic damage and marks them, E will proc this, marked enemies that are attacked take even more damage, and you and the ally get a 20% MS boost

with how spamable your E will become, you will proc this every time its up, and its CD per enemy so just keep spaming during teamfights. This will also help you chase down fleeing enemies with the move speed boost it gives your team.
Ionian Boots of Lucidity
CDR Not much to say here, CDR is important on Ashe, this gives you a ton, and extra movement speed, its highly recommended
Muramana
AD : extra damage on your E and autos (+6% added to abilities)
Mana : once tear and IM are complete mana shouldn't be an issue (all extra mana gives you more damage on your abilities once Mura is complete)
CDR
Passive: Muramana is great, as it converts both your mana and AD into on hit damage from both attacks and abilities. this will add an extra punch to your E's and will scale up with you.
Black Cleaver
CDR HP AD
Passive : Both passives will proc with your E, This means you are spaming out an AOE Armour Pen debuff, and gaining movement speed whilst being able to keep max stacks up on the entire enemy team during teamfights.
Chempunk Chainsword
CDR AD HP
Passive : Anti-Heal This will prevent sustain champs from healing teammates effectively, and make Lifesteal near useless.

Depending on the enemy comp/builds, you can swap chainsword out for items like Serpents fang for shield reaver, or umbral glave for the utility scanner, but chainsword offers the best stats, and should be considered before others
Vigilant Wardstone
CDR HP AD AP
Passive : Sup Ashe will gain a 12% bonus to all these stats, she is one of the few supports that can take advantage of all bonuses the wardstone offers.
Passive : stores up to 3 control wards, and increases your ward cap by one. allowing you to put out even more vision.
At full build this item gives Ashe a nice boost to all primary stats

Frost Shot : This is your primary kiting tool early, auto attacks will slow enemies briefly, allowing you to flee or chase.

Ranger's Focus : This ability isn't as needed on a support Ashe, it provides additional attack speed after 4 successive attacks, take a point in it for contesting and helping with objectives, but since you are spaming your E, even if you weave autos in between, you will not proc this most fights. Useful to have, but not what we are building her for.
Volley : Volley is the bread and butter of this build, with sufficient CDR you can spam this out as fast as your autos, it will provide an AOE slow to anyone hit, is an excellent kiting tool, and with this build, each volley will debuff anyone hit, while buffing your allies. With a range of 1200, you can safely spam it at a distance, from the backline, and watch as the enemies melt. Volley also has 100% AD scaling, and additional bonuses from your other items, so each hit will pack a punch.
Hawkshot : This ability allows you to give your team map wide vision, and locate the enemy jungle any time you wish. You can store up to two charges over its CD. Hawkshot will give your team a heads up on enemy jgl location, allowing them to plan ahead or play safe, and let your jgl know if its safe to invade. If it reveals an enemy it will give you an assist if killed over the next 10 seconds, so as a bonus tip, if you see a fight breaking out, toss a Hawkshot into a nearby bush the enemy may be running towards, if it detects them, you'll both help your team, and may snag an assist.
Enchanted Crystal Arrow : This is why Ashe support is broken. a map wide 3.5 second stun that will deal a ton of damage. This is great for engaging, fleeing, just to piss off someone as they respawn at fountain. This is by far one of the best engage abilities in the game, and with this build, at full, it is down to a 19 second CD.

Laneing Phase
Depending on the matchup, ideally you'll want to play fairly aggressively, your Volley will do a ton of damage, and is a great poke tool, with a long 1200 range to safely farm your support item. Try to aim between minions, and don't waste mana if you aren't getting support gold out of it.
Ashe is a strong poke champ, so use it to your advantage to deny the enemy lane the ability to all in you.

If you can stay in lane long enough without going oom, ideally you should have 950g by the time you do, first back, grab a tear, faerie charm, and boots, if you have extra get a vision ward, and if you dont have enough, grab what you feel is needed most, based on the matchup, tear + charm, or charm + boots

Your job in lane, is to zone the enemy with your Volley, Provide lane vision with wards, keep an eye on the enemy jungles location with Hawkshot, and if a fight breaks out, to dps the enemy at range, and keep all enemies slowed so your lane partner can escape if they need to.

Mid Game
Mid game goals are, Have Imperial mandate CDR boots & Tear completed, be at least level 6, have finished your support item. Once these are complete it is time to start helping other lanes.
Since Ashe in lane is fairly safe, with high poke damage, and mandate online you should be able to either all in with your ult, or play safe and keep them at bay with poke. Shoot some Ults into other lanes, Pinging the direction where it will land, most laners will understand this and try to position the enemy into the path of your arrows. This will provide you with free assists from across the map.
With mandate completed, the best spot to be in, is the backline of a teamfight. Try to contest any available objectives that are available, with your team.
If your ADC decides to wander off and farm a lane away from teamfights, prioritize your Hawkshots, to give vision, to that lane, so they don't get ganked by surprise.
Focus on building up the rest of your items, prioritizing based on enemy comp, generally Black Cleaver first, then Chainsword into Muramana, but if they dont have much healing, go Muramana after Cleaver

Late Game
By now, you have enough CDR to spam out your Volley without worrying about mana, and your ult should have a low enough CD that you can shoot it at fountain, and have it up again by the time you run to lane. You want to be looking for engagements across the map and shooting hawkshots, and Ult arrows to assist teammates. See whoever is most ahead, and be their backline.
Finish up your items, then sell your sup item for Vigilant Wardstone to complete your build.

What does Ashe support bring to the game?
Great early game poke
High AOE damage
consistent AOE Slows
AOE Armour Pen Debuff
AOE Magic damage Marks for her allies
AOE Grievous Wounds Debuff
Movement Speed buffs to her allies
Map wide Vision
and 3.5 second global stun on a 19 second CD


as an example:
Her basic E at 18, will do 80+(100%AD) physical +60 magic(IM) + 2.7%mana+6%AD(Mura) physical + apply armour pen(BC), grievous wounds(Chainsword), apply a mark +150 magic(IM) which gives a +20% speed boost to your teammates and spamming it, every 1.5 seconds at a 1200 range.


You will Buff, Debuff, and melt the enemy team, all the while knowing exactly where they are at all times.

Thats how you play Ashe Support.