Spread the sand to a depth of 3 inches in a sandbox made from scrap plywood or on a plastic tarp or sheeting. Lay the stones upright in the sand in a pattern and shape you like; the sand will hold the stones in place as you work. The stones should be packed together tightly, with parallel stones touching each other in their middles. Don't lay all the stones parallel, though—varying their direction will keep them from shifting out of place. (We created a double- spiral pattern, with one set of stones laid perpendicular to the other.) When you're happy with your mosaic, take a photo of it and note its size and shape for later reference.