Step 3: Constructing the basic shape
Take your two (or however many) boxes and line them up. In my example, I used two boxes, a full one and a half. I lined them up and marked where I wanted my final height to be at, then marked around the entire box with a marker.
After you have your lines made up and are satisfied with them, get out the craft knives and scissors and start cutting. Make sure that beforehand, you've taped the box so that both sides will stay together after they're cut apart.
You should have all the main blocky pieces of your tombstone now. Arrange them however you like and securely tape them together with the duct tape. Simple, right? Now get your trusty pencil and start drawing out the curves (if you want them). Again, once you're satisfied with how it looks, darken it with a marker. This time though, mark out the parts you're going to cut in order (as in number them somehow) and set them aside, you'll need them for the back.
Once you've cut the shape out on one side, flip the box-arrangement over and place all the cut out pieces on the boxes upside-down in the pattern you created to get an exact mirror image of the front. Outline these pieces with your marker and cut the rest out. You should be left with the base shape of your tombstone.