To build a sand pit, pick out a spot of the right size and shape for your desired sandbox then consider the following issues: drainage after a rain storm, shade for the kids who may play for hours outside, line of sight for the adults that are watching over the kids, the proper type of sand. Get a black plastic garden tarp and 2X4's or 2X6's that will fit the size of the area you need. Consider wood that has been treated and is also non toxic or synthetic decking wood. Dig a 3 inch deep hole in the squared area using a shovel and take your boards then put three nails or screws in from the top to the bottom. Lay the boards which by now should be in the shape of a square and then take the black plastic stuff then lay it on one side of the square to the other while doing this until the square is covered completely. Staple the plastic to the boards and flip the square over and set it in inside the hole. Finally fill in the sand.