Sketch the shape of the scenery you want to create on a sheet of plywood to create a theatrical flat. To make one that represents a tree, for instance, draw a tree shape, or for an urban scene, draw a set of rooftops. Mount the plywood on trestles and cut away the excess plywood with a jigsaw. Take off any rough edges with sandpaper.
Construct a right angled triangle from 2-inch by 4-inch lumber to make a bracing for your theatrical flat. Use a saw to cut the lumber to the correct lengths. The shortest edge of the triangle should be half the width of your flat. The next longest edge should be three quarters the height of the flat. Use 3/8-inch bolts to fix these two edges together at right angles. Measure the length of the triangle's hypotenuse with a tape measure. Cut your third edge from the lumber and bolt it to the other edges to form a triangle.
Place the middle length edge of your triangle bracing against the middle of the back of your flat. Position the bracing so it's flush with the bottom of the flat. Facing the back of the flat screw hinges on to the right hand side of the lumber and into the back of the flat. Use three hinges spread out over even distances. The bracing should now be fixed to the flat but fold up behind it when required. You now have a flat, ready for painting, that can be easily moved and stored, taking up minimal space. Simply unfold the bracing to stand it up when it's in use on stage.
Connect two flats together by screwing eye fixings along the edge of your flats and tightly passing a sash cord through each flat's eye fixings. Tie the cord to an eye fixing at the top and work the cord back and forth between each of the flats. Tie it off at the bottom. If possible, strengthen the flats further by running a batten across their backs. The batten should be made of 1-inch by 3-inch lumber. This can be screwed in place if the flats don't need to be moved during the performance.
Position flats on a stage with consideration of the audience's sight lines. Sit in the front row at the extreme left and right to view the scenery. From these positions, ensure that the audience members won't be able to see anything they shouldn't during a performance. If a flat's edge is unavoidably on show, consider hanging a simple drape over the flat to soften the line.