Building a perfectly square wooden box is more difficult than it sounds. If the box is to be a piece of furniture like a flower stand or an end table, efforts have to be taken to hide any raw edges of plywood. To build a box like this, you need to have all corners mitered. It simplifies the process, ensures clean edges and allows the box to come out perfectly square.
Set the table saw blade at 45 degrees. Miter all four edges on all six pieces of plywood. Peel off eight strips of masking tape approximately 6 inches long and gently stick them to the edge of the table, in a row for easy access.
Lay out four pieces of the plywood, with the grain running the same direction, on a worktable. Spread glue along the mitered edges of all four pieces in the direction of the grain. Stand up two of the pieces, bringing the mitered edges together.
Grab a piece of the tape. As you are holding the two mitered edges together, stretch the piece of tape over the two pieces at the corner on top, pulling to get it tight. Now tape the bottom of the two pieces together with another piece of tape.
Stand up another piece of plywood. Slide it into place, bringing together another mitered corner. Press together until glue squeezes out. Grab another strip of tape and tape the corners together top and bottom. Pick up the last piece of plywood, stand it in place, making the mitered corners come together and tape the corner top and bottom.
Place four strap clamps on the square frame one at a time and tighten lightly. The strap clamps will bring the box into square as they tighten. When all the corners are tight, finish tightening the clamps. Wait one hour.
Remove the clamps and set the square frame down across two sawhorses. Run glue along the top mitered edge. Place another piece of plywood into the mitered top, put some pressure on the top with your hand and then tape the corners, pulling the miters tight together.
Place the 14 inch strips along the edge of the top. Place the bar clamps on all four corners on top the strips. Pad the bottom of the clamp with rags to prevent breaking the miter on the bottom. Tighten all four clamps a little at a time, seating the miters together. Wait one hour. Take off the clamps, turn the box over, glue and clamp the top on using the 14 inch strips and clamps in the same manner.