A small list of cons ans pros:
pros:
-carry, so u deal lot of damage late game
-global ulti that can stun up to 3.5 seconds
-she can see in the brush soooooo far away=if u use that skill well(we'll see it later) u will be pretty ungankable
cons:
-she has only one damaging skill apart ulti and it will be most useless late game
-lowest base attack damage in LoL

For runes you should use the one yu usally use for an attack damage champ but if u have despersive runes or no runes, my hint is:
-mark: attack damage per level
-seals: flat health
-glyphs:mana regeneration
-quintessence: armor penetration
Probaly these aren't the highest damage runes, but these will help u so much, really.
For masteries, I improve offensive free spells then i go 21/0/9 taking mana per level and mana regeneration.
My free spells are exhaust and ghost, but tou can replace exhaust, so see nice spells and bad spells for ashe:
NICE:
-surge to back door faster and before your ulti, or even in a teamfight
-teleport: to be there when i'm needed
-flash: you haven't escape methods after all
BAD:
-clarity: u need it but not so much to use a spell slot
-promote: not, you don't need help farming or destroying turrets
-heal: easily not, you are a carry, u kill before they kill, so u don't need it

Now come to the most important part, items!
i start with a doran's blade, as all ad carry i think, then i buy boots of swiftness to have some nice mobility, but if u need the dodge take tabis, if u need tenacity take merc threads
my first item is: lifesteal, you absolutely need it! so some choices:
-the bloodthirster
-stark's fervor, but remember it gives no damage
-wriggle lantern, why not?
-bilgewater cutlass, my favourite for many reasons
-executioner's calling, even if i won't build crit chance
now attack damage:
-last whisperer
-infinity edge
-sword of the occult if you are getting lots of kills/assists
-tiamat, mainly cuz it gives even mana regen
-manamune if u can waste money to don't run out of mana
Now some defense:
-hexdrinker if you have issues with mages
-wriggle lantern if u didn't buy it before
-madred bloodrazor if u jumper one item before
and we end with something that i love: an hextech gunblade, so your ulti + surge=gay damage

I didn't say it before but my skill order is obvious: i get 1 points in all skills at level 3, then i max volley, then frost shot, sure your ulti and then your hawk shot.

I have finished, please leave a comment below ans remember: NEVER use Ashe in dominion, because in that map there aren't lanes, no lanes=no slow, no slow=no Ashe; u woun't even deal nice damage in dominion, so I've really finished, bye all!