Step 7/15:

This is the back part of the head on a sandbag. The hollowing is complete. It is important not to create air pockets that will be sealed up when the piece is reassembled.

Step 8/15:

Here I've perforated the inside with an awl in case I've left some areas too thick. This should be done to both halves but not too close to where the seam will be. If you go all the way through, it can be repaired later.

Step 9/15:

The edges of both halves that will meet should be scored with a fork as shown here.