After sanding Bob's face, I cut out the eyeholes and the mouth slot. It is very important to make sure your lines are straight for the mouth and the distance between the top and the bottom is the same width. Otherwise the mouth won't fit well.
I then glue the bottom part of the head back on to the face plate, leaving the back of the head off to add the mechanics later on. I use string or rubber bands to hold the head together while it is drying. I use Gorilla glue to attach the pieces.
together. Then I build up the cup around the mouth piece I removed. Some people use wood for the sides of the mouth because it is flush and will be more forgiving with the fit. I just make it out of magic sculpt and then use a belt sander to make the sides flush. I always build out wide so I can take off to make a good fit.