Measure out your 1-inch by 3-inch lumber. A standard sized theatrical flat is 4 feet by 8 feet long. The long, vertical pieces on the sides are called "stiles." These rails will be 8 feet long. The shorter horizontal pieces an the top and bottom are called "rails" and the horizontal pieces in the middle are called "toggles." These will be 4 feet long minus the width of the stiles. Do not assume that means taking 3 inches off each end of your rails. The lumber you buy will be slightly shorter than 3 inches wide wood is actually closer to 2.5 inches Measure it to get precise dimensions. Mark of the measurements on the wood with a pencil. In the end, you should have two stiles at 8 feet, two rails at (approximately) 3 feet 7 inches and one toggle at (approximately) 3 feet 7 inches. Taller flats will require more toggles, but for an 8-foot high flat, one will be enough.
Cut your lumber. Using your circular saw or chop saw, line up the blade with the pencil mark and cut. If you have made several cut marks on one long piece of wood. always recheck your measurements before cutting the next piece. This is because of kerf, which refers to the wood you loose due to the width of the saw blade. It could be as much as 1/4-inch and it could throw off your measurements.
Screw your flat together. Lay out the pieces of 1 inch by 3 inch wood in a rectangular shape. Take the 1/4-inch plywood. You will use this for corner blocks to hold the corners together and for keystones to attach the toggles to the stiles. You can use scrap pieces from past projects or you can cut new ones for your flat. The corner blocks should be a right angle triangle and big enough to cover the spot where the two pieces meet. Keystones can be rectangular and the same width as the toggle (In this case, about 2.5 inches.). Press the rail and stile together to make sure they meet and put your corner block on top. Using your drill, screw the corner block into place. An "X" pattern of screws will keep it sturdy. You'll need two Xs of 5 screws each to keep the corner secure. Repeat this with the other three corner blocks and the two keystones.
Cover your flat. Take a piece of muslin, a soft fabric very common in theater set design, that is slightly larger than the flat. Pull it over the flat and staple it to the back where the corner blocks were just screwed in. A staple every 6 inches is plenty. As you go around the flat, make sure to stretch the muslin tight in order to eliminate sags and puckers in the fabric. Your flat is now ready to be painted.