A human skull has a unique shape. Making one out of clay can seem tricky at first because there are a few areas where the main shape is defined by specific contours, and if you aren't exactly sure how to create the basic structure, you'll run into problems later in the sculpture. Luckily, there is a technique you can use to achieve an accurate finished product without using too much effort or time.
Create a sphere with some polymer clay. The amount of clay will depend on how large or small you want the clay skull to be. Roll the clay in your hands until you have formed a smooth ball.
Roll the sphere in your hands, concentrating a little more on one end to create an egg like shape. Point the smaller tip of the "egg" towards you and press two indentations on each side to create the cheek area.
Press the rounded area in between the indentations flat. This will be the area where the teeth and nasal openings will be added. Turn the skull to the side and use a straight edge to create a flat "ledge" where the jaw area and the rounded back part of the skull meet.
Use a clay carving tool to gently score the outline of the teeth over the lower half of the flat area between the cheek indentations. There are two ways to execute the teeth; outlining each tooth separately to form two rows or simply create a strong line across the mouth and using a scoring tool to make vertical lines across it.
Add the nasal opening. It looks like an upside down V, and it should be added in just above the teeth.
Use a tool with a rounded end to press the clay in circle-shaped indentations where the eyes would go. Depending on how large or tiny your skull is, this tool could be anything from the end of a ballpoint pen to the end of a rolling pin. Smooth the indentations with your fingers so that the sockets look natural to the shape and not forced, especially around the edges.