I'm a low-elo scrub who's abusing Comet Ashe to climb.
Here's how you can do the same!

Ashe is one of the strongest if not the strongest ADC in the early game. Additionally, she is able to stay relevant in the late game without a lot of items thanks to her high-utility and low cd ultimate.

Comet Ashe makes Ashe stronger in the early game and more useful in the late game in exchange for her late game dps, and is a perfect choice for autofilled ADCs who don't want the pressure and responsibility of a carry. (If you int you are still somewhat useful).

Arcane comet is good at helping you win lane hard so if you do get really fed during early game you can always change your build to DPS.

Strengths:
Strong early game
Does not require lots of resources (kills & gold)
Ult can turn teamfights
Ult allows for picks

Weaknesses:
No mobility
No dps

Arcane Comet is the key rune here. It allows Ashe to outtrade all ADCs in lane (maybe except Senna but frick that champ).

Manaflow band gives you mana. I went with this rune because the other runes are pretty useless on this build.

Absolute Focus gives you extra AD, which helps you in lane. Transcendence can also be considered here if you are considering stacking lots of CDR (if you are building Cleaver + ER).

Scorch is the best option here, as you are trying to max out early game power.

Inspiration: Take Biscuit + Time Warp Tonic if you are playing Comet Ashe ADC. This gives her extra sustain in lane against a poke matchup. (enemy Brand/Lux/Senna).

Domination: Take Zombie Ward + Cheapshot if you are playing Ashe support. Zombie Ward + Umbral Glaive gives your team extra vision and gold and AD for yourself. Cheapshot allows for more damage in lane.

Umbral Glaive: Cheap AD and allows you to clear wards. A core item for support Ashe.

Black Cleaver: A core item imo if you're playing ADC. Phage passive allows you to kite and chase enemies and the health is useful against assassins.

Essence Reaver: Alternatively you can build Essence Reaver as it provides good mana sustain. I don't like it personally because I want to play a more supportive and utility role as Comet Ashe.

Youmuu's: Gives you CDR and gets you back to lane faster. Since you are not a priority for gold and kills, you want cheap AD. Also has CDR

Ionia boots: Cheap and gives you CDR.

Mortal Reminder: Useful against tanks and healing comps.

GA: You probably won't need GA unless they have a dive comp because you can do damage from really high range.

Edge of night: Defensive item.

Redemption: Support item.

Comet Ashe excels at poking and zoning her opponents in the early level. Since you're also stacking so much CDR, she also has access to her ultimate stun every teamfight at level 11.

When to pick Comet Ashe:
- When you're autofilled and you have no idea how to play ADC
- When your team has a lot of carries/assassins (Fiora top, Yas Mid, Shaco Jungle, Quinn Top, Kha'Zix Jungle, Qiyana Mid, Pyke Support, Vel'Koz Support you get the idea.

When not to pick Comet Ashe:
- When your team actually needs a source of DPS (4 tanks/ babysitter support/ supportive jungler/mid/top)

Early game, look to constantly push the wave, zone your opponent and get turret plates or kills or Dragon.

In the mid game, you can look to rotate mid and constantly push the wave if your midlaner is better at sidelaning but if your midlaner needs to catch those waves constantly, try to push out sidelanes with jungler or support and pressure Herald or Dragon. If a skirmish or teamfight breaks out, your ultimate is great at shutting the enemy carries of tank out of a fight.

In the late game, you will have your ultimate up constantly so use that to your advantage. Remember you don't deal any damage to tanks but your W poke will still hurt squishies. Try to position yourself so that you can constantly shove waves and siege towers but if a teamfight break out, stay back and use your CC to disable enemy ADC/assassins.

As Comet Ashe, you are agreeing to trade carry potential and DPS for early game dominance so don't play safe, take all the resources and look to outscale enemy ADC. Look for agressive trades, try to be useful with your CC and prevent the enemy ADC from getting any resources.