There are a few ways to do this. If you are just trying to grind them up really fine, then it will be much easier. If you are trying to extract into a concentrate, then this can take a little work. Especially if you intend to consume this.
As for powdering the leaves... You will want to wait until they dry, then send them through a coffee grinder, or good blender. You can always do it by hand, but this will take a while.