Summarize:

Canvas frames are also sometimes referred to as stretcher bars, which feature pre-cut slats that you can slip together to create the frame. This is the easiest way to customize the frame to fit your purposes and put it together quickly. Most professionals use stretcher bars. DIY frame kits like EasyWrappe are prefabricated in a variety of different sizes that will allow you to quickly fix a canvas to a frame with no outside tools, other than an X-Acto knife to trim the excess canvas. While you have to pick from a limited variety of sizes, you can stick the pre-cut wood bars to the canvas and fix them in place in less than five minutes. Get enough canvas to fit around the frame by at least six or eight inches on each side, depending on the width of the frame. It's essential that you have enough overlapping canvas to attach to the back side of the canvas, or the stretching won't work. Measure the dimensions of the frame you have, or of the eventual canvas that you want and purchase or cut a larger size of canvas, accordingly. Unprimed, sometimes referred to as “ungessoed” canvas is easier to stretch than primed or “gessoed” canvas. For the best results, buy ungessoed canvas and prime it later. You'll need a few basic tools to complete a canvas stretching job properly. Try to get the following supplies:  A spray bottle filled with plain water. It’s common to dampen the back of the stretched canvas. As it dries, it will shrink and thus tighten the canvas even more. Gesso. This primer is commonly used to treat the canvas after stretching. It's a white paint mixture, made of gypsum, chalk, and other ingredients, available at most art supply stores. Specialty pliers used for canvas stretching. Available at most art supply stores, canvas pliers have flat surfaces that can be used to easily stretch canvas without pulling holes in the material. Staple gun. Regular desk staplers are not sufficient for stretching canvas. You need a heavy-duty staple gun to plant staples into wood and carpentry staples capable of gripping into the frame. The canvas should be cut several inches wider than the dimensions of the stretcher bars, taking into account the width of each side of the frame, as well. You'll need this extra canvas to have something to get a grip on to be able to pull and stretch it. After you've got all your supplies, frame and canvas, cut the canvas to shape using a very sharp utility knife, or an X-Acto. Tearing the canvas will make a straighter line than cutting it will. Get the cut started along the appropriate line using your knife and consider tearing along the grain to make an accurate shape.
Purchase a stretcher or build a frame. Get enough canvas for the job. Get the other necessary materials to complete the job. Cut your canvas.