I can't tell if you are asking how to trim it, or when to trim it. If the latter, usually you wait there's some amber trichs, and more often than not most of the hairs will have turned red/brown. If the former, besides the above, it's really up to you. Some people only remove fan leaves, some trim every little piece of leaf they can get to so the bud is primarily bracts and the stems they are attached to, and everything in between.