You can slush silicone yes, but it will never sit right and look as good as cored silicone masks. Here is a run down for that process:
You need a core to sculpt on (lifecast)
sculpt your mask paying attention to the thickness of the clay as this will dictate the thickness of the rubber later on,
then you mold your sculpture, fiberglass is a good choice or epoxy if you can afford it.
If its a full head, a 3 piece jacket is easiest. Make the front one piece and the back half 2 pieces. Be sure to add enough bolts to keep it all tight, one through each eye will help also
once you've removed the jacket, work out the volume of the clay you scraped out to give you an idea of how much silicone you'll need.
Then you need to add a tube to the inside of the core that allows the silicone to be injected, preferably through the top of the head. Then you can pour your rubber down the tube and into the mold.
Thats a general overview but what you really need is a good dvd like this one:
Neill Gorton's Creating Character Prosthetics in Silicone
Expensive? Yes but not if it saves you wasting $100 worth of silicone.
There was a guy on here who did a Hellboy mask and he laid his process out very well, I'd do a search for that thread and have a read. Its a big undertaking.
