If you find that commercial pre-stretched canvas is too expensive or if you are just interested, you can make your own canvases. You can buy rolls of primed or raw canvas in various sizes and qualities. Once bought, the first thing to do is to stretch a canvas of a desired size using stretcher bars.
The stretching process is fairly straightforward. You will need to buy stretcher bars. You can find them in any serious art supply store or on the Internet. Stretcher bars have tongue-and-groove corners and are usually 1.25 or 2.5 inches wide. You can order them in various lengths up to 72 inches.
To make one canvas you will need 2 sets of 2 equal bars. All four bars should fit together tightly and no nails are used. After you have assembled the stretcher, use a T-square to check if the four corners are square.
Now, stretching a piece of canvas involves the following steps:
1) Center the stretcher on top of a piece of canvas that is 1.5 inches larger that the stretcher on all sides.
2) Fold the canvas around a set of two opposing bars. Using a tack (usually included with each set of bars) fasten one side of the canvas to the center of the bar.
Then stretch the canvas by hand or with canvas pliers and tack the other side in the middle of the other bar. A straight line in the form of a crease running from one tack to the other should now be visible.
3) Next, repeat the procedure of 2) with the two other opposing sides of the canvas. A diamond shaped pattern should now be visible.
4) From here on, you continue from the middle of one of the bars and add a tack every 3 inches or so until you reach the corners. At the corners, nicely fold one end of the canvas under the other end and fasten this corner piece onto the bar with another tack.
Do not trim excess canvas. Instead, fold it over the back of the stretcher and tack it down just in case you need to re-stretch the canvas in the future.
5) If you notice any wrinkles in the canvas, you can remove one or two tacks in the right places, pull the canvas taut, and replace the tacks. To remove small uneven areas in the canvas, moisten the back of the area and let the canvas dry.
6) Usually the stretcher bars come with a set of so-called keys, i.e., small wooden wedges. These keys fit into slots at the inner corners of the stretchers and if necessary will further tighten the canvas. However, it is best to wait and see if the canvas actually slackens over time. If so you can pound the keys into slots at that time.
Note that nowadays, most artists use staples and a staple gun to fix the canvas to the stretcher. This approach is quicker and more convenient. However, for the sake of permanence and stability, it is still a good idea to use a regular tack in the middle of each of the bars.
Finally, when a painting is finished you may want to loosely place a sheet of cardboard inside the back of the stretcher. This will protect the back of your canvas.
Once you are used to the procedure, stretching a canvas goes fairly quickly. It is quite a bit cheaper than buying pre-stretched canvas. However, there still remains the task of priming the canvas in case you bought a roll of raw canvas. But, as far as stretching a canvas in concerned, this is all there is to it.