check your local library for books on special effects any good one should have it in it. Basicly the most common way of doing it is to take alginate cover the area you want cast in this case the whole head being carfull to avoid the nostril opening when the alginate has set start making a layer on top of this with plaster bandages on just the front half. when this has set put a lot of vasoline on the edge closest to the back of the head and than covering the back half of the head with plaster bandages and overlapping the front a little where you put the vasoline. when all has set pull of the back plaster bandages, with a spatula cut down the middle of the back of the alginate and slowly pull the persons head out. That's about it if you want more detail let me know I can write something up for you but like I said a lot of books have a section in them about lifecasting.