Measure the length of the canvas frame. Divide that number by 2 and mark the center point of the frame's length.
Measure the space between the two lengthwise pieces of wood at the midway point. Cut a piece of wood to fit that measurement.
Test the brace by sliding it between the long pieces of the frame. It should fit snugly.
Screw your L-brackets into the brace and frame.
Drill two holes on either side of the frame so that there is a hole from the outside of the frame to the inside or end of the brace. Space the holes away from the edge of the wood of the frame to guard against splitting the material.
Fasten the brace with two screws on either side. You may remove the L brackets at this time or leave them for extra support.