The one I've seen is a Mardi Gras mask. Cut a paper plate in half. Glue (or staple) a craft stick to the lower corner of the flat side. Cut a slot to look through. Cut a 3" (or so) Styrofoam ball in half, glue glitter or paint with glitter paint, glue to paper plate just above the slot, as two eyes. Hold mask in front of face. If desired, paint or glue other face features on mask. Staple two craft pipe cleaners to the top of the mask as antenna.
I have also heard that some people use a large Styrofoam ball as a base to make a papermache mask, which is then removed.
See also, "How do you make a mask out of a sheet of Styrofoam?"