First, I liberally apply Vaseline to the sculpture as a release agent. Then I mix Celluclay (a kind of commerial paper mache pulp) and roll it out into approximately 1/8" thick sheets between two pieces of wax paper. These I put into the freezer for 8 minutes to make them less sticky and easier to work with, and then I peel the wax paper off and apply the sheets over my sculpture. I blend the edges of the sheets together and smooth out any major lumps, but otherwise I don't fuss too much over the texture. It's much easier to deal with after the Celluclay is dry.
The cast will need to be cut into pieces in order to remove it from the original sculpture. After I apply all the Celluclay, I make score marks along the parting lines with a clay tool. Otherwise, the Celluclay would be very difficult to cut apart when it is dry.
Then I set the mask in front of a fan to dry. Usually after an hour or two the Celluclay will start to feel leathery and I'll be able to add a little more detail back in with a wooden clay tool. After a day or two it is usually dry enough to remove from the sculpture.
Here's the beginnings of the cast, with the score lines pressed in:
Then I tie the pieces together using twist ties (I save these compulsively) and apply glue all along the seams:
After I've gotten a sturdy shell on which to build, I begin to add texture and refine detail, both by adding back on with wet Celluclay or by carving with a Dremel. Here's the rabbit mask with the beginnings of hair texture, created by adding a thin layer of wet Celluclay and working it with a wooden clay tool: