Summarize the following:
Using a saber or keyhole saw, cut out the bottom of the bucket. Use 40 to 60-grit sandpaper to sand the edges until they are smooth. By removing the bottom of the bucket, it will be easier to lift the bucket off of the sand.  If you do not want to remove the bottom yourself, then ask the hardware store if they can do it for you. Alternatively, use a regular bucket without the bottom removed if you do not have a 1 gallon (3.8 l) bucket. You can purchase 1 gallon (3.8 l) buckets, saws, and sandpaper from your local hardware store. Fill a third of the bucket with sand. Add an equal amount of water to the sand. Pack the sand down with your hands. Repeat this process until the bucket is full of compact sand. Remove the bucket. Once the sand feels solid enough and does not give under pressure, it is compact. Ask you create the layers of sand, place the bucketfuls of sand close together. Fill in gaps between the bucketfuls of sand with more sand and water. Pack the sand with your hands until the base feels solid. Keep adding layers of sand until you have reached your desired height. Once you have reached your desired height, use a sand wedge hoe or a plastic knife to smooth the surface of the sand flat.  Large sandcastles are typically 5 by 5 feet (1.5 by 1.5 m) or bigger. If you plan to build a sandcastle taller than you, then bring a step ladder to the beach so you can reach the top.

Summary:
Remove the bottom of a 1 gallon (3.8 l) bucket. Build layers by placing the bucket upside down on the base. Cover the entire base with a layer of compact sand. Add a second layer to create more height.