Making stage flats is essential in most sets for theatre and film. For the most part stage flats are covered by luann, which is a thin composite wood. However, luann is incredibly expensive and canvas may be the better option for the budget. Perhaps, luann isn't the better choice because you wish to paint very intricate designs on the flat and wish it to be canvas for its more authentic look.
Cut down the lengths of 1-inch by 4-inch wood into strips: two 8-foot long strips, three 3-foot 10 1/2-inch long strips. Lay out the strips of wood on the ground. Lay out the three strips of three 3-foot 10 1/2-inch long strips and put the two pieces of 8-foot long wood perpendicular to the three strips of wood. Assemble the two outer 3-foot 10 1/2-inch long strips so that they are at the ends of the 8-foot long pieces and the middle strip is in the middle of the 8-foot long piece. The end result is something that looks like the figure "8".
Apply glue on the ends of each piece of three 3-foot 10 1/2-inch long strips and position it where it was before. Staple three narrow crown staples through the 8-foot long piece into the ends of the lengths of three 3-foot 10 1/2-inch long strips. Staple three staples into each side of the frame until the crossbars are secure in the frame.
Stretch a large sheet of canvas over the frame. Wrap the canvas over the top length of the flat; make sure the canvas' weave is congruent with the vertical and horizontal directions of the frame. Staple the canvas to the frame and cut off the excess. Stretch the canvas over the flat to the opposite side of the flat. Staple the canvas to the frame and cut off the excess. Choose a side and stretch the canvas to that edge, staple it and cut off the excess; stretch it to the opposite side, staple it and cut of the excess.