When I was learning casting, I had the advantage of getting to talk to my sculptor buddies. When it comes to 1:6 scale heads, molding couldn't be easier, really. It's the other stuff that requires a real skillset to do well.
I make all of my molds as one piece, and I use Smooth-On rubber. The biggest trick to making a good mold, and subsequently, good castings, is pressure. Pressure Pots are essential. I know you can piece together a pressure pot, as I've read about it in this great book, "Pop Sculpture", however, I bought my pot a few years ago. It's a big investment, but a necessary one if you want quality work.
We can talk more about it in private, if you'd like Anzik. It's not too difficult to mold and cast heads!
Trev