For Jack's head, we sewed a large white felt ball and made the facial features out of black felt. The pattern and instructions came from here:
http://www.thesilverpenny.com/FeltBall.html, but enlarged so one piece measures 20 inches from tip to tip. My wife used a back stitch to put it all together, sewing the ball inside-out for a more finished look. Once turned right-side out, we stuffed the head with batting, leaving room to stuff around the neck once attached. Instead of stitching up the opening at the bottom of the ball, we hot glued a circle over the opening and used a razor to make a cross hatch big enough to fit around the neck piece. We didn't tape, glue or sew the head onto the neck, so we were able to take it on and off for transport and general spookiness ("And because I am dead/I can take off my head" -Jack).