Step 4: Draw the gravestones as vector images
The laser cutter works by burning away a thin layer of material in a single plane on the surface of a sheet of acrylic. By burning away successive layers, you can gradually carve away a 3D object. This is quite a limited approach to 3D fabrication, as it cannot produce complex objects with overhanging features. What's more, it's very slow; each pass of the laser may take several minutes but only remove a tiny fraction of an inch of material depth.
Shade your 2D vector images according to the different depths at which you want to etch them. In my images, I used darker shades of grey to indicate where I wanted more material to be etched away (except for the cilantro stone, where I deviated from this scheme for the sake of contrast).