You might want to post pix to give us a better idea of what you're working with, but for shaping fur-on skins the basic procedure is below.

Lightly dampen the skin side of the hide. Repeat until it is soft. Once softened, the hide is moulded onto a form to help hold the proper shape. Eye hole and ears are tweaked into proper shape. Allow to dry.

Once dried, you can make a permanent form for the mask using any number of materials. There is an artist-quality paper pulp that dries VERY hard and is quite light. The name escapes me for the moment.

When shaping the face and head-form, you'll probably find it helpful to reference photos of the animal in life, although in most cases you'll want the mask to be mounted flatter than a traditional taxidermist would do, as you're not trying (in most cases) to make it look like it's alive, but just to retain semblance of it's original form.

Taxidermy eyes can be had quite cheaply online, although people's opinions on masks with or without eyes vary widely.

I'm by no means an expert taxidermist, I'm just experimenting with masks myself right now. There are others on this board who have made several full masks. A quick forum search for "full mask" will reveal a great number of results.