I have some excellent step-by-step notes on this that one day soon I hope to put up on the web (I know, I know, that's not terribly helpful right now).
For basic foam shapes, one approach is what's known as "the wedge method". If you do some searches here on the forum you should find old discussions that discuss this in detail, but the basic idea is that you create a round head shape out of a series of wedges (try to imagine the skin of an orange cut in to about 8 slices). By varying the number and shape of wedges you can create different shapes.
You can find a
n old wedge pattern I made up a long time ago on Puppets and Stuff.
The Foam Book decribes two methods of patterning heads, "The Three Piece Head Method" and "The Nip and Tuck Method". There are also videos available that demonstrate those techniques. You can order the Foam Book & videos from Amazon or Grey Seal Puppets -
http://www.greysealpuppets.com
A really good source of information on patterning is the first video in the "From Sketch To Show" series from Playsoup. You can order it online from
http://www.playsoup.com
I've also collected a lot of
useful puppet building information that might help you on Pinterest (Pinterest is a good place to search for puppet building inspiration and information).
The best thing to do imho is try several different approaches and find what works bets for you. As Slackbot said, there is a lot of trial and error involved. Good luck!