Draw a preliminary sketch of the set with graph paper, a pencil and a ruler. This sketch takes into account where the characters will make their entrances and exits and whether there are windows, platforms or staircases. For this example, the entire play takes place in a library which has an upstage wall full of books, a downstage left free-standing doorway and a downstage right short staircase.
Measure the number and size of flats needed to construct the upstage wall and a downstage right wall by the staircase. A flat is a basic scene unit that consists of a large wooden frame covered with canvas and supported from the back with triangular braces made of wood and metal. Nail the frame together first, then use a staple gun to attach the canvas securely to the frame while the entire thing is on the ground. Construct the support braces and attach these to the back of each flat. Lift into place.
Construct the freestanding doorway by using the same techniques as building a flat. The doorway can either be supported from the back, or the left and right ends of the frame can be stabilized by perpendicular blocks resembling an upside-down T. If an actual door is needed, use plywood, hinges and doorknob hardware.
Create the illusion of stairs by building three or four sturdy platforms that are each the same width and 7 inches high but which vary from 8 to 10 inches in length. Stack and align them at one end with the longest platform on the bottom and use metal braces to attach them. Construct a second set of stairs to position behind the one visible to the audience so actors can make their backstage entrances and exits. Place a flat near the higher side of the steps to suggest that the staircase ascends higher.
Prep the canvas with a light coat of gesso. This is a standard white paint mixture that helps to shrink and tighten a new canvas and fill in the pores. Apply two splatter coats in a neutral shade. This pointillist approach is what will give your faux wall a sense of texture and depth. When dry, proceed to paint the desired scene. Using the library example, this would include bookcases, portraits and the illusion of a hallway leading to other rooms.