There are numerous reasons to make a foam skull, from science projects to holiday decorations. For a science project, form the skull and label each section as necessary for the project. If it's going to be a party decoration, you can make it look any way you wish.
Glue pieces of foam together to form a cube, if you cannot find a large enough piece for the skull. Allow the glue to dry completely before carving the skull shape.
Carve the top of the foam block to the shape of a head, using a serrated knife. For realistic skulls print a picture of a skull, using life-size measurements. Tape the picture onto the block and carve around the outline. Tape an inverted view of the printout to the other side of the foam block. Use the two pictures as a reference while carving.
Hollow out the eye sockets, the nasal cavity and mouth. The eyes are actually located in the centre of the front of the skull. Print a picture of a face and tape to the front of the skull for reference, if needed. Carve out the jaw bone and the teeth in detail.
Sand the carved skull with a fine grit sand paper so the finished surface is smooth. Sand in small sections to avoid deforming the skull.
Make a thin glue by mixing craft glue with water in a ratio of one to one. Paint the skull with the glue mixture and cover the skull with strips of tissue paper. Smooth the tissue paper with your hands so there are no air bubbles or wrinkles.
Add cracks in skull with fine tip marker. Use the skull picture as a reference for placement of the lines, which represent joints between skull bones. Define the teeth with the marker, as well.
Soak a tea bag in warm water for about a minute. Squeeze out the excess water. Dab the tea bag over the skull to give it a more natural colour.