This is how I used to cast my heads. I would then reinforce the head with fiberglass. It made a very strong head, but it is very time consuming.
For someone wanting to make one or two figures/ heads, this is an easy and fairly inexpensive way to cast a head.
For more information on building your own figure you check out my earlier posts and also check out a wonderful book called, "Figure making can be fun" by Mike Brose. Mike has written this great how to book which he sells on his web site http://www.puppetsandprops.com/home.html
Al Stevens has also written on not only figure making, but mechanical installation which can also be found on Mike's web site. It is located under the "Fred Project"