Prepare the material. The standard theatre flat is 4 feet wide and 8 feet tall. This design can be customized for other sizes, but the principles will be the same. Cut the plyboard into one foot squares, and then cut those in half to form triangles. You will need six triangles per flat. Cut the 1 x 4s. You will need 5 of them for each flat: Two cut the width of the flat, 4 feet. These will form the top and the bottom. Cut two to the height of the flat, minus 7 inches for the two 1 x 4s on the top and the bottom. For an 8 foot tall flat, that is 7 feet 5 inches. Finally cut one length of wood that will form the center slat. This will be the width of the flat minus 7 inches again. For this flat, that will be 3 feet 5 inches.
Place the wood for the frame. Lay the framework out on the ground. Place the two sides, the top and bottom, and the center slat. All should fit snugly. Make any necessary adjustments at this time. Then, place a triangle over each joint, for a total of six, two at the top, two at the center, and two at the bottom. These will hold the frame together.
Assemble the frame. Screw each triangle into the frame, making sure that each strut is affixed to the triangle in at least one place. Start at the top and work your way down, making sure that each joint is snug. The side with the triangles will be the rear of the flat.
Cover the frame. Cut a sheet of canvas 6 inches bigger than the flat on all sides. For a 4 x 8-foot flat, that would be 8 feet 6 inches by 4 feet 6 inches. Using the stapler, affix one side of the canvas to the back of the flat. Wrap the canvas around the front of the flat, pulling it taut, and then affix the top and bottom. Pull out any slack left and affix the other side. Cut off the excess canvas.
Finalize your flat. Now you can put on the finishing touches. Several flats can be attached with hinges and set up and slight angles to support themselves. You can attach them to any flat surface with just a few screws, or even hang them from cables to form flies, scenery than can be raised and lowered from offstage. You can paint them as needed, and repaint when necessary. If you plan to move them frequently, placing simple handles on the rear of the flats can be a great benefit.