Mark the boundaries of your sand area with a chalk line from a building supply store.
Create a concrete curb by placing the 2x10 form boards around the perimeter of the sand area, about 6 inches higher than the top of the poured concrete. The boards will help keep the concrete from escaping your sand area.
Hammer the stakes into the ground at equal intervals (about 12 inches) using double headed nails. The stakes should be directly against the 2x10 form boards to hold them steady.
Dampen your sandy area and use a rake to smooth it out. The smoother the sand is, the smoother the concrete area will be after it is poured.
Spray the form boards thoroughly with diesel fuel so the concrete will not adhere to the wood. The diesel fuel must be completely dry before continuing.
Using ½ rebar rods, construct a grid on your sand area, forming 16-inch squares inside the boundary. You will need to lay one layer vertically and then another layer horizontally.
Tie the rebar pieces together with rebar wire, so the squares do not shift when you pour the concrete on top of them.
Pour concrete onto the sandy area, keeping it inside the form boards.
Allow the concrete to set for three or four days, and then remove the 2x10 form boards.