This is a question that comes up often on the forums. All you need is a wireframe consisting of 4 entities. Then make a coons surface. That will give you half of the neck transition.
...or you can hire a monkey to bang on your keyboard for a few hours until it looks just right. It should only cost a few bananas. I'd try the wireframe method first unless you happen to know a zookeeper. The 4 entities are as follows:
1) A line which is the thickness of the headstock, where your thumb would be.
2) An arc that would be on the neck where the palm of your hand is. It's about a quarter of a circle.
3) A spline that is part of the edge of the top of the headstock.
4) A spline opposite that one. Make it tangent to the center line of the surface of the neck and tangent to the surface of the back of the headstock.

It's probably best if these 4 entities are closed but not entirely necessary. Then make the coons surface. I like choosing the parabolic option for a surface of this nature but it's up to you. When you get good at this you can make one coons surface that has the neck and both transitions at either end of it.
Best of luck to you.