the best way to do it is to cut round the tail while its still attached, get 2 6in long twigs the thickness of your thumb. put one twig under the tail the fox side of the cut round the base of the tail, put the other twig on the top of the tail again the fox side of the incision you have made. squeeze the two sticks together with one hand one side of the tail the other hand the other side of the tail, and pull towards the end of the tail while holding the back end of the fox firmly in place with your foot.
You can still do this with the tail unattached, clamp the base of the tail bone in a vi