The armature is bolted together in the approximate shape, wrapped with some wire and then the clay is applied. Using a hammer to put the clay on speeds things up. The mold doesn't have to be very smooth or accurate.
As you can see the eyes are an undercut which will prevent the mask from being pulled off the form. You can slide a knife up between the mask and the mold and cut them off before lifting the mask off. Then you just have to reattach them before making the next mask.