Place the two-by-fours so that they are on their sides in an upright position, perpendicular to the surface the stage will be built on. Establish the perimeter of the stage with four of these boards. Use a measuring tape to determine the width and length of the stage, and the locations of the boards that will be bracing its deck. Lay out the boards so that they are spaced evenly apart and so that they form the rectangular shape of the stage. Nail the boards together.
Measure the distances between the bracing boards. Cut some two-by-fours down to the same length so that they fit in those areas. Slip the boards in and nail them. These will further support the stage deck. The usual spacing for support boards, similar to floor joists, is 16 inches.
Attach the plywood floor. Cut the plywood sheets into sections as needed and nail them down to the two-by-four frame. For the easiest job, size your stage at 8 by 8 feet, in which case you can use two full-size sheets of plywood.
Lay down a finish if desired. Sand the wooden floor and brush on wood treatment oil if the stage will be outdoors. This will prevent rain from rotting the wood. For indoor stages, consider a smooth overlay such as linoleum.
Create a support for a stage curtain by attaching 2-by-2 or 4-by-4 inch posts to the front two corners of the stage with wood screws or nails. Attach another 2-by-2 inch board at the top, stretching it from one post to the other, to create a support for a stage curtain. A dowel works perfectly for this, since two shower curtains can then be hung over it for a theatrical stage curtain that separates in the middle. Build a similar structure to support backdrops. Painted backdrops can be laid up against the structure.