BIGGEST PIECE OF ADVICE: Wear rubber or latex gloves! This stuff is insanely sticky and instantly bonds to whatever it touches. For the love of all that is good, wear gloves!
Also, do this outside, duh. Sez that right on the can.
I covered a large garbage can with waxed paper and then applied some "large gap" expanding foam insulation. It took about 4 cans ($3.50 ea) to do a complete skull, 6 ribs and 2 femurs. I thought I shouldn't just lay out stripes for the ribs, but built them up in several squiggly passes...maybe that'll make them a little less likely to crack immediately?
SECOND PIECE OF ADVICE: If you get this sh-tuff on you, do not attempt to 'dust' it off of your hand with dirt. This forms a semi-permanent dirt putty that will take days to wear off. Not even the beloved baby wipes help.
Next, I sprayed out an outline of a skull on more waxed paper and started building it up. Can't really build up too much 'cause it expands and sags a bit, but you can rough it out. I left some gaps for the nostril and eye socket and built up some areas for horns.
I let it dry for about 6 hours and then carefully removed the waxed paper to let the bottoms dry. Watch out for wet spots! Seriously! Occasionally, little blobs broke through the surface and expanded and dried. But the last step was refining the shapes, carving out the eyesocket and nostril, cutting off any blobs, etc. Done. Well, apparently, not all foam is a loverly fossil-shade of ivory like mine was. If it's a wonky color, spray paint. Then, Really Done.