dip the wound in pottasium permanganate to help it clot, get the cardboard roll from a bog roll, cut it from top to bottom place it over the tail and make the correct size for your dogs tail, then gaffer tap it from the top to the bottom including about an inch onto the hair the tip of the tail should be inside the tube, about an inch is fine, this will let the air get to it so it can scab over and the tube will protect it whilst it heals. keep an eye out for infection and bobs your fanny just out of interest do you feed comlete dry food diet?