Cut your canvas to the dimensions you desire. Cut 6 to 8 inches wider than your stretcher bars (sides of the frame) to have some slack to work with.
Lay your frame on top of the canvas and center it. Line up the grain of the canvas parallel with the stretcher bars, so they are squared.
Staple the canvas to the frame. Begin with the longest side of the canvas. Fold it over and put three staples in it towards the center of the stretcher bar. Move to the opposite side and do the same. Pull on the canvas tightly enough to make it taut like a drum.
Staple the other two sides. Go to the first unstapled side, fold the canvas over the stretcher bar, and give it a good pull before stapling it. Go to the last side that needs to be stapled and give the canvas a really good tug before putting in the staples, ensuring it is firmly wrapped around the frame.
Staple the corners of the frame. Working from the center of each stretcher bar outward, continue to fold and stretch loose pieces of the canvas over the stretcher bars, adding staples until you you have about 4 inches of of unstapled space at each corner of the frame.
Fold the corners and staple them down. Pull on the corners tightly, as this will be your last opportunity to tighten the canvas.
Check your work. Flip the canvas over and tap on it with your finger. You should feel a drum-like tension when you tap on it. In other words, the canvas should have little bit of bounce to it, but not be as stiff as a board. It's good to have a little bit of give, because the canvas will shrink a little after the final step.
Hammer each the staples down so they are flush with the wood and and securely embedded in the wood.
Gesso the canvas. Using a bristle brush, apply a coat of gesso to the canvas. Use your finger to get rid of any bubbles in the gesso. As the gesso dries, it will cause the canvas to shrink even more, as well as add a smooth protective surface to the canvas. Dry for 30 to 40 minutes.
Sand the canvas with a piece of fine-grain sandpaper. Lightly spot sand areas that are rough or lumpy by moving the paper in a circular pattern.