Begin with the lower right hand corner. Pull the canvas up tight on the bottom edge of the stretcher bars and tug the corner to the right. Staple the canvas to the lower part of the frame an inch from the corner. Relax your grip on the corner of the canvas.
Fold the loose corner in toward the center of the canvas so that the ninety degree corner of the canvas makes a square with the ninety degree corner of the frame. The canvas will hang over the right side, but it should be neatly tucked in on the bottom edge.
Place a finger on the inside corner of the frame, holding in the bottom pleat, then pull the right side of the canvas over. This will fold the ninety degree corner of the canvas in half on itself. The right side should now be pulled taut against the frame, creating a neat corner.
Staple the corner and the canvas on the right edge. Turn the frame and repeat for the other three sides.