Materials for 6'x6' Sandbox:

(2) 12-foot 2x29 lumber wood 
(1) 5-foot 2x6 
(28) 3-inch deck screws
54 cubic feet of builder's or mason's sand (or 3/4 cubic foot per square foot of sandbox area)

(*Note: Use pressure-treated wood, but if you're nervous about toxicity problems, you could use untreated lumber — just don't expect it to last forever.)

To Make the Sandbox:

1. Level the ground and remove obstructions.

2. Saw four 72" 2x29s (if your lumber is 1/8" short, don't worry about it). Screw the frame together, using four 3" screws per corner to make a sandbox that's 73½" square on the outside. Countersink the screws so that they get a good "purchase" on the wood.

3. Saw two 2x6s to 28½" for the angle braces, with 45-degree miters on both ends.

4. Hold the braces in position and screw them using three screws per joint.

5. Fill the box relatively full of sand.