Construct a plan or blueprint for the size of the scenery mural or backdrop. Typically, these are elongated rectangles. The planks of wood you get depend on how big you want your scenery to be. Have these cut at the lumber supply to your specifications. Sand these down, so they are less likely to cause splinters.
Assemble your plywood boards in the shape of a rectangle, with the third short board in the center, as a support. Place the two diagonal boards in opposite corners, to keep the scenery fabric flat.
Attach metal gussets to each of the joints on both sides, using wood nails to affix the gussets to the plywood boards. Stand the scenery flat up to make sure it can stand freely. Lay it back down flat, and then stretch a fabric canvas tight over the surface, leaving a little excess to nail to the back of the flat. Nail down the canvas before painting it.