Step 3: Sculpt Mask and Claws
First make an armature base for the ears by scrunching up aluminum foil and "sticking" them to your head using tape and plasticine. Using your oil-based sulfur-free modeling clay (plasticine!) cover your cast with a thin layer of clay. You want to be sure to cover beyond where your mask will end. Block in details with more clay. Sculpt away to your heart's content, using your sculpting tools, fingers, and some vaseline to smooth everything down.
Be aware of undercuts, which will complicate your casting and molding stages. I have 'free-standing' teeth, and instead I should have made sure there was clay behind the teeth. When I made my mold and cast, I ended up having to chip and dremel pieces of resin out from under the teeth.
Don't forget to sculpt a claw while you're at it! (The last picture shows a claw alongside two other sculpts from another costume project. Free cookies if you can guess from what!)