Determine the size and number of flats required for the production. The most common size is 4 by 8 foot, although they can be constructed larger or smaller as required. The only change in these instructions would be to buy the correct length of lumber for your project.
Lay out the lumber on the floor to form the wall that you need. A flat wall is not heavy, so laying out a square with 2 or 3 vertical struts should create a wall strong enough for your needs. When finished, you'll have a square with two or three vertical struts placed at regular intervals inside the square.
Nail or screw the lumber together and attach the vertical support struts. Insert a corner block inside each right angle and attach it to both pieces of lumber. This will make the wall a bit more rigid and less likely to sway or move during use. Attach the corner stones in each corner. These triangular-shaped thin pieces of wood will help to hold the corners at the correct angle.
Stretch the untreated muslin over the top side of the flat and nail into place. The easiest method is to begin at one end of the flat by attaching the muslin all the way across the piece of lumber. Pull the muslin tight when you reach the first strut and nail it in place. Repeat the process until you have attached the last piece of lumber.
Paint the muslin surface with sizing paint to properly prepare the material for painting. Watered down paint can also be used for this process, although you run the risk of having a blotchy, uneven surface if you are not extremely careful.
Allow the flat to dry for at least 12 hours and they can be painted to the requirements of the stage production.