Choose a flat area for building your sandbox. Although most sandboxes are in the full sunlight, slight shade might make it more enjoyable during the heat of the summer. Use an area where the wind will not hit the sandbox. Trace around the outside of the pool with a spray paint or even a garden hose.
Shovel out the dirt the depth of the pool plus a couple of inches. The lip of the pool needs to be level with the ground so children do not trip over it, causing the plastic rim to break.
Add 1 inch of sand to the bottom of the excavated area. This saves the plastic pool from cracking against a rock or hard ground. Level it over the area with the back of a garden rake.
Drill several 1/2-inch holes through the bottom of the plastic pool with an awl or drill. Any rainwater that collects in the sandbox needs a way to drain or the sand will not dry out.
Place the pool on top of the layer of sand. Fill the interior of the pool with playground sand until it is level with the ground surface.