For these skulls I used "JOVI" air-dry clay, however polymer clay could be used.

Start of by rolling a ball of clay between your palms so that their are no cracks in the clay. The clay tool that will be used is shown here.

Using your thumb and index finger, pinch the clay to form a 'chin'.

Create an indentation for the eyes using the back of the paddle end of the clay tool.

Create two nose slits and teeth slits by using the sharp end of the clay tool.

Using the sharp end of the tool, press and scrape the clay to make deep eye sockets.

Using a little bit of water, smooth out any cracks and leave to dry for the recommended time suggested by the clay manufacturer. Here are three I have completed

Paint 'em up! Use black acrylic paint to paint the eye sockets and teeth and allow to dry before painting the rest of the skull silver. Once dry, coat with a layer of varnish/gloss.
Now your skulls are ready to use!