Measure and cut a piece of canvas that overlaps the edges of your stretcher bars by 4 inches on each side. Lay the canvas face down on a table. Place the stretcher bars centered on top of it.
Fold one side of the canvas cloth over the edge of the stretcher bars. Place one staple through the canvas fabric and into the wood at the center of the fold. Do the same for all four sides on the canvas.
Pull the corners of the fabric taut under the stretcher bars. Pull the corners of the canvas onto the stretcher bars' corners. Fold the edges of the canvas over the stretcher bars, creating an inward facing triangle of fabric on the corners of the stretcher bars.
Staple the canvas fabric to the stretcher bars by placing staples at 1-inch intervals along the sides. Pull the canvas tight around the stretcher bars to make it maintain the proper shape.
Place canvas tacks through the fabric and into the wood along the edges of the stretcher bars. The tacks add an extra bit of security to keep the canvas from detaching from the stretcher bars.