Take a blob of clay the size of a large pea and press it onto the forehead of the skeleton.

Press the front down to create the basic plane of the face. Press the back down and around to form the back of the head.

Smooth it around a bit until it looks something like this. Remember to make sure that this clay is pressed firmly down so that is sticks well.

Here's a front shot of the same thing. Don't worry too much about the exact form, but try to get it to look like this.