I included a wiring diagram, which will come in handy for those of you who don't have a lot of experience with LEDs. I used a 6 volt power source, not 3 volts like in the diagram, but the wiring is the same. I used a 10 ohm resister. It gets hot but so far I have not had any problems.
1. Use Great Stuff to fill the crack on the first tombstone. It expand so don't use too much.
2. Drill holes through the eyes of the skull that are just large enough to tightly hold your LEDs in place. Wire your LEDs appropriately, then pull them through the front until the LEDs are just barely protruding from the sockets.
3. Prep the skull and second tombstone for great stuff. I used some 1 inch thick pieces of wood to hold the front of the tombstone off my work bench, and then put a large piece of aluminum foil under the skull to keep any excess foam from sticking to my work surface.
4. Fill the gap between the tombstone and skull with foam, and stick the cut out pieces back where they came from before the great stuff dries, this will hold them in place.. Make sure the LED wires don't get buried. I also put a little extra Great Stuff on the front of the grave, so it looks like it is dripping out.