Article: You need 2 4-foot (1.2 meter) boards from 1-by-3-inch (20 by 65 mm) or 1-by-4-inch (20 by 90 mm) of pine lumber. The finished flat will measure 8 feet (2.4) tall.  Lumber dimensions refer to the unfinished boards, so 1-by-3-inch lumber actually measures 3⁄4 inch (1.9 cm) (19 mm) by 2-1/2 inches (64 mm). And, 1-by-4-inch lumber actually measures 3⁄4 inch (1.9 cm) (19 mm) by 3-1/2 inches (89 mm). If you're using 1-by-3-inch lumber, the stiles should be cut to 91 inches (230 cm) (2.31 m); if using 1-by-4-inch lumber, cut the stiles to 89 inches (230 cm) (2.26 m). Don't attach the boards to one another. One board will become the toggle. Place this inside the frame between the two stiles so it divides the frame into equal top and bottom sections. Two boards will serve as corner braces. Cut these on the miter and place them between the top rail and left stile, and the bottom rail and left stile. Attach them to the 4 corners of the frame, where the rails meet the stiles, with carpenters glue and pneumatic staples. Use these to attach the corner braces to the rails and stiles and to attach the left side of the toggle to the left stile. Use glue and staples. Attach this to the right side of the toggle to the right stile with glue and staples. Lay the cloth over the frame and staple it into place along the insides of the rails and stiles. Paint the boards with thinned carpenter's glue and smooth the edges back do wn.  It will stiffen and shrink slightly, becoming taut.
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Cut lumber for the rails, which will form the top and bottom of the flat. Measure and cut 2 boards to form the sides of the flat, called the stiles. Assemble the boards into a rectangle on the floor of your workspace. Measure and cut 3 additional boards from the same lumber. Cut 4 triangular pieces of plywood to serve as corner blocks. Measure and cut 5 straps from the plywood. Cut a trapezoidal piece of plywood to be the keystone. Turn the frame over and cover the front with muslin or canvas. Fold the edges of the cloth back so the rails and stiles are exposed. Go over the edges with a damp sponge, let the glue dry, then trim the cloth. Cover the cloth with a coat of paint to size it.