Select the size of flat you want to build and draw a plan.
Cut the plywood according to size. For a rectangular flat, you will need two long sides or stiles, two short sides or rails, and one or two toggles the same length as the rails.
Screw the stiles and rails into a rectangle. Use the framing square to make sure the right angles are exact.
Screw the plywood corners onto each corner, 1 inch inside the edge of the frame. This will support the corners and make the frame sturdier.
Depending on the size of the canvas you will use for the face, you may add one or two toggles. These wood rails inside the frame are screwed parallel to the rails. You can use them to help secure the canvas in place, especially if you are using more than one piece of canvas. They also help strengthen the frame. For a flat 8 feet tall and 4 feet wide, add one toggle across the middle of the frame.
Screw the keystones into place on the toggles. These plywood pieces should be 1 inch narrower than the toggle and they attach across the joint between the toggle and the stile, running parallel and directly behind the toggle. They support the toggle.
Staple the muslin to one side of the flat.
On the stapled side, glue the muslin to the flat underneath the stapling.
Stretch the muslin as tight as it will go and staple and glue the opposite side of the flat.
Repeat Steps 1 to 3 for the other two sides of the flat.
Size the canvas. Sizing is a powder that is mixed with water and painted on the canvas. This will tighten the muslin more and make it waterproof so you can paint it. When the sizing dries, paint your backdrop on the canvas.