You can use a comb or a stencil to make a line and carefully follow it. If you have good dexterity and flexible mirrors, you can do it by eye. Or if you fade to

You can use a comb or a stencil to make a line and carefully follow it. If you have good dexterity and flexible mirrors, you can do it by eye. Or if you fade to 0 length you may not need to cut a sharp line depending on your style.