This is a simple effect that I honestly didn’t think would turn out as good as it did when I was thinking about giving it a shot.
Once I had the form, I laid thin (pulled-apart) pieces of Halloween spider webbing on top of the skull, and painted on a layer of liquid latex to keep it all stuck together, and to give it a bit of a mummified effect.
Finally, I airbrushed on some acrylic paint for detail (although if you don’t have an airbrush, I suspect that light, misted coats of spray paint would work). The entire process took me less than a half hour (outside of the overnight drying time for the latex), and cost me virtually nothing. Great, cheap effect for the holiday season!