Heads start out as a hand-sculpted solid clay sculpture. This design is molded in silicone and the final (wearable) head form is cast from resin. An internal armature is added so the head can be worn and the jaw can be opened/closed.
Vision is usually through mesh near the eyes (not through the eyes themselves), through the mouth, or under the chin.
The glass eyes are painted (and eyelids hand-sculpted over them for the correct expression) then the final shape is created by carving/gluing/adding flexible foam to the base structure.
Once the basic head structure is complete, the fur and other details are added.