Measure the width of the 4-foot boards. They should be approximately 2 1/2 inches.
Measure 2 1/2 inches or the width of the 4-foot boards from each end of the 8-foot boards and mark the distance with a pencil. Measure 3 feet from the top and bottom of the boards and mark another 2 1/2-inch section.
Set your blade on the table saw to 3/8 inch high and cut out the marked portions on each of the 8-foot boards.
Measure the width of each 8-foot board and mark this distance on both ends of each 4-foot board. With the dado blade still set to 3/8 inch, cut out the marked portions. The 4-foot boards should now fit like puzzle pieces into the grooves you made in the 8-foot boards.
Lay the 8-foot boards on a flat surface and fit the 4-foot pieces into the grooves. Square the corners with a carpenter's square, then glue the pieces together. Let them dry thoroughly.
Drill a pilot hole through each joint with the 1/16-inch drill bit, then drill 3/4-inch screws through each hole.
Staple a 3-yard piece of muslin to one corner of the flat. Draw a line of glue on the top beam, then stretch the muslin taut across it. Staple it on the other corner. Repeat this process until the fabric is stapled to all four corners, then add additional staples as needed to secure it.
Attach a piece of muslin to the back of the frame, if desired. Allow the glue to dry thoroughly.
Trim off the excess muslin.
Water down white paint or primer, then brush it onto the muslin. Let it dry completely. This tightens the muslin onto the flat and prepares the surface for painting.