Wrap the subject's head tight with a roll of cling wrap and then apply clear tape all over the cling wrap. The goal is to make a solid casting mold. Use a marker to trace your subject's hairline. Add an extra half inch all the way around. This allows work room.
Remove the skull shell. Cut around the hairline with a pair of scissors and place the shell on the foam head model. Trace the edge with a marker.
Remove the shell and rub a light layer of petroleum jelly on the head. Dab a thin layer of liquid latex on the foam head with a rubber sponge, covering the area enclosed inside your tracing. Let this dry completely and then repeat the process four more times.
Let the cap harden for 24 hours before you remove it. Use powder on the inside and outside of the cap as you remove it from the foam head. This will prevent it from sticking to itself. Be careful removing the cap. It's very thin.
Paint the shell to achieve the look you're after. You can use white and burnt sienna for a Caucasian skin tone. Blending other colors will produce different results.
Put the cap on your subject and use makeup or liquid latex to blend the cap into the subject's face to achieve a realistic transition. This will give you an effect that is hard to tell from the real thing.