Prep your tombstones first. Most are made of a hard foam substance, which is pretty easy to paint. You can purchase glow paint, along with a thin artist brush, at most craft stores or on line.
Make sure to read the package directions on the paint before purchase, in case there is a note about how the paint may react when applied to a material similar to your tombstone. For example, some spray paints can "melt" Styrofoam.
Also, think about weather conditions in your area. If you are in a rainy location, you may want to pick up a sealer to apply over the glow paint. Again, check the sealer's package directions for notes about product compatibility.
Make sure the surface of the tombstone is clean and free of dust.
Paint the letters and image details, like the whites of eyes, with the glow paint.