I start out with 4 pieces. A front, a back, and two shoulder pieces (which are trimmed to fit) The width of the front and back pieces are determined by the circumference measurement around the neck opening of the mask. If you're following along at home, take a dressmaker's tape measure and measure around the opening and cut the two main pieces. The front and the back CAN both be the same size, but together their size should equal the circumference of the neck. Pin them on your suit to make sure the top edge matches up (where it fits to the mask), you can see my white back piece has a curved upper -- that's so it matches up with the back of my mask.
I sew down the front to the back on both sides, approximately 1 inch, though depending on the mask it might be a little more.
Now I add in the shoulder pieces! Sometimes these need to be played with but it is always a triangular insert to open up the neck at the shoulder for good coverage so your human neck doesnt peek out when you do that inevitable puppydog tilt-head-at-sound move that is pretty cute but usually results in skin showing. Give yourself a good several inches at the bottom of that triangle to make sure your shoulder has room.
I feel this is an important step, try it on! If you can move your neck all around (and your head fits through) and there are no huge wrinkles or ugly folds, you've got a good neck. Try to twist it around so the back faces front and look at that side, too. You want it to lay fairly flat over the front and back of yourself, and the shoulder inserts make room for your shoulders, of course. Trim up the bottom edge so it has a neat appearance, and now you're ready to attach it to your fursuit head!
I then sew the neck to my fursuit head, when sitting on a dummy head the shoulder inserts stick out a little. Brush out those seams and you're done!
If you find you've got some unsightly folds, but you can't reduce the size of the neck fabric without affecting the fit, I like to weave in some elastic thread to make the neck stretchy. Follow this tutorial for more info on that subject: http://fursuit.livejournal.com/4065775.html
On this particular fursuit, I wove in a small 4-inch patch of elastic thread on the back of the neck flap to help it curve in while still laying flat.
Hope this clears up the neck process! As always, I am happy to answer any questions!