I responded to your original post thinking you were using an artificial head, so I apologize for wasting both of our time. If you search through the archives, it is there. There are several methods. Cutting the back of the head is one, as a matter of fact, one that Brian (duxrus) uses. I cut around the bill as if I was going to skin it and replace it with n artificial. The fleshing is much easier in my opinion, and simple to do. Rebuilding the head is something I rarely do any longer, but I think it is something you should learn. Cougar highlighted the finer points. Basically remove all the brain matter and flesh, and rebuild it with clay or epoxie sculpt. I prefer the rigidity that sculpt offers. After removing the head, you can trace the outline both top and side, and replace this "flesh" with whatever you wish. I try not to cut the skin whenever possible, although I certainly did when I started. I like to be able to inject clay=based wallpaper paste through the ear hole to allow me to fill the crop, neck and head of the bird to allow for added plumpness and fill areas that may need some filling.
I would really consider using an artificial head though I think you should learn multiple methods. The artificial head can cost a bit more, but you will put in more man hours, and wind up with a slightly (sorry Brian) I repeat slightly inferior bill do to some shrinkage. I repeat Brian it is slight, and some artificial heads certainly have their flaws as well, but are in the long run, much easier to use. Tony Finazzo sells heads pretty darn cheap, and they are just fine. Around 8 bucks.
If you would take more time, and search the archives, you will probably find your answers several times.