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If you want your sandcastle to be 5 by 5 feet (1.5 by 1.5m), then outline a 5 by 5-foot area. Use a stick or shovel to mark the outside perimeters. Use large buckets, shovels, and garden spades to pile the sand over the entire area. For the best results, use sand that is located below the tide line. This sand is typically wet and compact. Alternatively, mix one part sand with one part water in a bucket. Use this sand to create your base. Use a stick to poke a grid of holes into the base of sand. Slowly pour buckets of water over the entire base. Let the water drain all the way through the sand. Pour one bucket of water for every 1 foot (0.30m) of sand. If the sand still feels too loose or dry, then add more water. Press on the sand with your hands to pack it down. You can also use your feet to pack the sand down. Pack the sand down until it feels like a solid block beneath you. You can also roll a large bucket over the sand to pack it down. Use your shovels and buckets to pile more sand onto the base. Poke holes into the sand with a stick. Pour water into the sand and pack it down with your hands and feet. Keep adding sand and water until you have a solid base of sand that is at least 1 to 2 feet (0.30 to 0.61m) high.

Summary:
Outline an area as big as you want your sandcastle to be. Pile 6 inches (15cm) of sand evenly over the entire area. Pour water into the sand to make it compact. Use your hands and feet to pack the sand down. Add another 6 inches (15cm) of sand.