Here are a few links to some general information & construction. Best I can tell the secret is heating the material to the proper temperature and constructing your model form correctly.
http://www.halloweenfear.com/vacuumformintro.html
http://members.aol.com/GCGassaway/vacuform.htm
http://members.aol.com/KMyersEFO/vacuum.htm
http://www.warmplastic.com/
http://www.hhhh.org/~joeboy/resource...cuumtable.html
http://www.build-stuff.com/