Cylinder leak down test

While looking at your page. I noticed that you told someone to run a cyl. Leak down test to determin if they had a head gasket or head problem. How does one run such a test? I have a 97 4.6 sts cadi. That over heats. Cant keep coolent in it. Motor runs great no misses. And do you know where or what web site I can go to to find info. W/ pictures showing how to tear down engine to make repairs. I have a chilton manual for this year model, but its not very clear as to location of parts to be removed. I have made a donation, hope it will help you guys.