Ashe is an ad carry who mainly specializes in slowing the enemy down and finishing them off. She works best with going bot and might need a support to make her stay longer in her lane. I recommend Leona, Janna, or Sona. For her build, try to focus on attack damage, critical damage, and life steal, as well as getting some attack speed. Since her passive gives her critical chance, critical damage and critical chance help to stack and to do much more damage.

At the start of the game, Ashe is has low mobility and has little attack damage. However, she makes this up with the slows she produces with her frost shot and volley, and also a stun with her ultimate. She is amazing at poking the enemy in the beginning of the game. She does not do well with ganks though. Since she is very squishy, many people like to gank her or to run in to get a kill. Be very cautious for ganks when playing her.

At the start of a game, her passive is a great way to get some early damage on an enemy champion. Make sure to make your first hit in a game to be on an enemy champion since it would do critical damage.

In mid game, make sure to place wards on the dragon and baron nashor about 15 minutes in the game. This could prevent ganks if you are getting them, or your team could be aware of them getting the dragon or baron and rush in to gank them. Mid game is probably the best time to farm. Make sure to get plenty of minions and you could probably jungle a bit if you want.

Late game is the time to show the other team what you're made of. When you complete your build. Don't be afraid to rush in to get a kill. Of course though, make sure not to dy. You could spam your volley since it would have a short cooldown and a lot of mana. Work with your team to help them out and win the game.