Use stolls recipe for paper clay, use that to model a witch face onto a foam wig stand.
Make hat out of paper mache, use cardboard or chicken wire to build upon.
PVC pipe or wood for frame, build up using batting, foam, etc. Rots Skull Witches were built on paint cans, so use what you have, obviously Stoll uses anything and everything for armatures.
Mache paste on fabric or monster mud overtop, or just use fabric.
The rest is just stylization, paint and trial and error. Best wishes and Happy Haunting!