It's not to hard at all!
Your going to need some type of foam. It has to be a closed cell type, something that doesnt let air flow through. Neoprene rubber is good. Another good solution is the foam that meat comes on, the stuff from the grocery store. It's usually nice and flat, and if you slip the meat guy a $5 he might give you a boxfull of it
You are also going to need some vaseline or di-electric grease. You'll use this to pack your processor slot/socket pins.
You dont need to insulate the back of your motherboard. It doesnt usually get cold, unless your doing some really heavy duty cooling...
Depending on how your block mounts, you will have to do it a little different. But all your trying to do is keep the stuff that gets cold from having contact with ambient air.
I dont have my camera right now so I cant show you an example, but here are some other sites that have good instructions and pictures on sealing your CPU up...
Benchtest.com
I get my camera back in a day or two so I'll whip up something and take some pictures for you...