I needed something to approximate the foam thickness inside a helmet as well as become a barrier between my head and the cast tape. Cheap closed-cell blue camping mat works very well. (about $7 at Kmart) Whatever is left over can be used for tweaking outfitting in boats.
Cut 3" strips out of the foam.
Wrap one around your head like a low riding head band and cut it to size.
Next, place the fitted band on your head and do your best Karate Kid imitation. Be sure to ride it low. You'll cut away ear space and eye space later. The idea is to get the correct diameter of your brain bucket. You'll get the shape of your odd mellon later in the process.
The second 3" strip is to make a measurement of the top ofyour head. Measure it. Cut it. Attach it to the headband. Viola!
Take the whole thing off your head. You need to put some kind of protective barrier over your hair. A latex or silicone swim cap is perfect. Since I didn't have one of those, I just stretched a latex glove over my head. (Foghorn Leghorn anyone?)
Wrap the head band with 6 layers of cast tape, and put the thing back on your head. Be sure to squeeze all the excess water out of the tape so it doesn't drip in your eyes or on your clothes.
Let the tape harden before you remove it. You'll have a perfect shape copy of your head.
Repeat the process with the strip on the top and let it harden.
Gently inflate a balloon inside the the crazy looking contraption. This will give you a mock head inside the thing so you can cast other pieces of tape over top that will create good structure for the bondo.
Let everything dry overnight.
The next day you can use bondo to smooth out the bumps and create a custom design on your helmet design.