I found some unused insulation/Styrofoam in a box that was a good size. I cut out the rough shape of a tombstone, and drew some designs on them with a sharpie. The first one was pretty basic--R.I.P. and a couple cracks here and there (Have I mentioned the lack of artistic skill? Yeah--there's none). The second one I tried some Celtic designs, some skull and bones crosses, and a more elaborate outline.
Next step--fired up the glue gun and started tracing over the marker in hot glue. The main thing to remember--especially for the letters, is to go slowly, and squeeze until you have the width you want. This creates a more seamless line, as opposed to going back and forth over the entire length. I made that mistake on the 'R' and it turned out kind of wonky.
The spider on the second one was actually pretty easy--though the skull and bones were a little difficult. The Celtic pattern was very easy, and I was wishing I had done a more elaborate design.