Step three: Push the second container into the cement. If needed, fill in the sides of the container with cement, and leave the skull to dry. After about 6 hours, when the cement is firm but not completely set, carefully remove the container that is in the center of the skull.
Note: You can also choose a container that is small enough the fit in the skull and just leave it there permanently, if you prefer.
Step four: When the cement has dried, remove the duct tape from the sides of the plastic skull to remove the cement and reveal your design!