it. At first it will be difficult to gauge how low a minion's HP needs to be for a last hit, but with practice it will come naturally to you. Employing this technique will ensure that you have a healthy amount of gold for the first time you return to base to shop.
As Ashe, your target gold amount for your first trip into the lane is about 2200. This will allow you to buy basic boots (which can be found under the Movement shop section), allowing you to outmaneuver your opponent, and a BF Sword (found under Attack → Damage) which will significantly increase your damage output and make you much more dangerous to enemy champions. Once you have 2200 gold, disengage from the minions, stand next to your tower, and bluepill (B key). This will return you to the shop, where you can purchase the items. Then, you can activate your summoner spell Teleport by clicking on its purple icon, select a minion, and immediately go back to what you were doing.
You want to continue to farm in this manner throughout the course of the match. Later on, you will be able to inflict much more damage per hit, so it will no longer be necessary to wait for their hp to get low. Also, as the game progresses it will be more and more useful to farm with Volley, since you will want to quickly take out a wave of minions, get their gold, and get back to defending other tactical objectives.