I thought a Green Man mask would look just right on the staircase wall in my house, so I sculpted one in Chavant clay on top of a plaster cast of a friend's face.
When the sculpture was good enough for me I built a clay wall around it and poured in DragonSkin Q silicone. The silicone was slushed around over the sculpture until it set (in about seven minutes)
I applied some more silicone, with added thickener, to cover all areas properly. I tinted the new layer green so I'd see exactly where I'd put it.
I put a reinforcing plaster shell over the silicone. This shell created a "cradle" to put the silicone mold into, making sure it maintains its shape while the casting process is going on.