A vet has to do it. Do not attempt it yourself unless you are instructed by a professional breeder or a veterinarian. Your puppy could bleed to death or die of infection if you do a backyard job on it.
How Do I Dock A Miniature Schnauzers Tail?
This has to be done at birth, by a Vet. It is too painful to do it later on, similar to you getting a finger cut off.