How-to: Head to your local fabric store and buy enough fabric to cover the canvas (use the dimensions of your canvas plus 3 inches more on all sides). Stretch your fabric around the canvas and hold it in place as you staple the fabric to the back.
Start with a few staples in the center of one side and continue with the opposite side and ends, gently keeping the fabric taut as you work toward the corners. Fold the corners under as you would if you were wrapping a package.