I've been doing this for a while. You need to get a 3d program. I chose Rhino as it seemed to be pretty popular. The nice thing about Rhino is that it will handle DXF and DwG files too. I downloaded the tutorials and then started to play with the various commands needed to create a neck. There is a tutorial on officialluthiersforum.com that is pretty useful. I start out with a perimeter of the neck and then build up from there. Once you have a handle of the commands it is pretty easy. The hardest part is creating the surfaces that blend together. While I'm sure that I could improve if I knew more about Rhino, I'm pretty happy with the way my necks are turning out.