Slide your mitred stretcher bars together. Each side should be equal to that of the opposite bar, so that the corners are square. Check your angles by measuring each side. Adjust as necessary until your sides are equal and angles are square.
Cut the canvas three inches larger than each side of your frame.
Pull the canvas tightly. Staple the canvas to the stretcher bars in each corner so that it is taut and evenly spread across the bars.
Secure the canvas more by adding more staples. To do this correctly, canvas pliers will come in handy. Move from side to side, directly across, while holding the canvas taut and staple, being sure that the canvas is not sliding or moving. Move outward towards the corners.
Staple the corners down while being sure to keep the canvas tight and free of ripples. When the canvas is tight, fold excess canvas at the corners over and staple into place.
Ensure that the canvas is secure throughout the whole frame. Make any adjustments that are necessary.