Step 3: head cast
Obviously, you'll need the head mold from the previous step for this step. Beyond that, you'll also need more plastic wrap and some of the air hardening clay.
The head mold by itself will do little to help you create your new head. We need something sturdier. That's why we, in this step, will create a head cast.
The cast will be made from the clay. Since we don't want the clay to stick to the plaster mold, you first need to cover the mold in more of the plastic wrap, mostly the inside. The wrap will function as a 'release agent'.
After you've covered the mold, start pressing the clay into it. Don't make the layer of clay too thick; it'll take (too) long for it to dry. Don't make it too thin, either, because then the resulting cast won't be sturdy enough. Furthermore, make sure all of the clay is properly blended into itself.
After you've covered all of the inside of the mold with clay, wait until the clay has fully dried. Check it every now and then. Air hardening clay has the tendency to shrink somewhat while drying and you may have to smooth it out somewhat more.
Once your cast is ready, move on to the next step.