Usually, when shaving your head with a no-guard clipper or with a razor, it may take a few times, but most of the time, you just have to go by feel. It's a little hard doing it holding a mirror in one hand, and clippers in the other, but some people have a three-door (mirror) medicine cabinet and they just fold the two end doors in at an angle which allows them to see the back of the head real good...Or you can just get the help of a friend/buddy. Of course you can always shell out a few bucks at a barbershop.