Hey
I have done this - I seem to remember posting about it in one of the other similar threads..
Anyway, I bought each material separately - alginate powder and silicone rubber.
As already mentioned here, the temperature of the water you mix with the alginate is quite important, too hot and it will set instantly. With cool water the one I used took about 4 minutes to set. A bit of vaseline stopped it sticking to me (a trim of hair helps too!) and I used an empty plastic soda bottle with the top cut off as a container.
The silicone just needs the hardener mixing in and then pouring into the alginate mould, and then left overnight to cure. Then the mould is pulled off and voila!..it was a PERFECT copy, very detailed and textured to the last pore
Actually I'd love to make another one or two but the cost of shipping the materials out here is a bit expensive.. we'll see
G
P.S. I think I have somewhere the website addy of the UK company I ordered from if anyone wants it - I'm sure the same materials can be found in the US too.