Sculpture Casting.

Once the faces where sculpted out completely and smoothed down, I built small walls around them in MDF to form the walls for the mould. 

I used a simple 1:10 platsil silicone, poured over the sculpts after mixing up and dispersing the air bubbles. The silicone was poured into one empty corner of the mould, held from a height in order to help make sure there where no air bubbles too. I also glued the walls down so they were easily removable afterwards; I wanted to make sure that the silicone mould could be pulled off the top of the sculpt once it had set, in case the mould didn't turn out correct the first time. This way I would have a second chance at getting the mould correct in case it did go wrong, (preparing for the worst case scenario, as my time management would have been strained if I had to resculpt any of these character faces.)