The proper way would be to go to the taxidermist that you plan on using and let him/her do it. Other than that have them show you how to do it.

There is more than just the neck measurement. Measure tip of nose to front orbital ridge of eye, tip of nose to back of skull (not used too much any more) and the circumference of the neck at the base of the skull with the skin off. A good taxidermist will want to measure the angle placement of the rack on the head from the burrs to the eyes, some use the tip of the beams to the nose.

It takes a lot of proctice to properly cut the earbuts, eyes, tearducts, lips and nose away from the bone. It only takes one slip to mess it up.