Second photo by the very talented Ming Wu
Step 1: What you'll need for the brain
To make the exposed brain you'll need:
-Brain shaped Jello mold (I got mine from http://www.thinkgeek.com)
Step 2: Make the brain
Mix a few drops of red paint into some liquid latex to give it a pink colour. Remember that the latex will dry much darker.
Pour a small layer of latex into the mold. Slowly build up layers of latex until you you have the desired thickness for the brain prosthetic.
Allow the latex to dry completely. This will take a very long time. Be patient.
Step 3: Trim to fit
Cut off the straggly edges and then trim the prosthetic until it fits comfortable on the side of your head.