You'll need a staple gun, a stretched canvas (sold in art-supply stores), and some fabric, preferably heavy linen or cotton in a pattern without straight lines. Cut out a piece of the fabric that is four to six inches longer and wider than the frame.
Place the material pattern-side down on your work surface, and center the canvas facedown on top of it. Then staple the cloth to the back of the frame. Start by placing one staple in the center of the left side of the frame. Pull the fabric tight, then do the same on the right side. Continue stapling on opposite sides, keeping the fabric taut as you work out from the center toward the edges of the frame. Repeat on the top and bottom sides, and make neat hospital corners when you reach the edges.