Silicon Head cast / replica

Hey Friends.
I remember a Guy that try to use 1:6 silicon Heads for to give them facial expressions. Someone of You had Expirience with a cast of a plastic Head and the use of silicone ?

I've never tried that myself. I'm building up the courage to modify a Phicen into a mermaid using silicone, but I haven't quite figured out how to do it.