Cut the frame pieces. For a 4 foot wide by 8 foot high flat cut two pieces of 1 by 2 inch lumber to 8 feet and three pieces to four feet.
Cut dado joints using a dado blade on a table saw. The dado joint removes half the wood from each piece of wood where the two members of the frame will interconnect. For a 4 by 8 flat, the frame will be around the perimeter with a 4-foot center brace in the middle. Layout the frame and mark the dado joints.
Connect the frame pieces. Interlock the dado joints and connect using a screwdriver and 5/8-inch wood screws. Brace the frame with triangular pieces of plywood attached at the corners of the frame on the back. Use 1/4-inch plywood and 5/8-inch screws.
Cover the framework with muslin. Start on one side and staple, using a staple gun, the edge of the muslin to the back of the frame. Wrap the muslin around the frame, pull tight and wrap the cloth around the frame before stapling on the back of the frame.
Paint the muslin. Paint causes the muslin to contract and produces a tighter cloth cover.