For this part of the tutorial I've used an old support with the canvas removed. The hardboard corner plates were chiselled off as hardboard resists staples.
For stretching canvas you need only a staple gun. To make life easier, invest in a pair of canvas pliers which won't damage the canvas.
Your hand built support. Canvas. Plenty of staples. Primer, and (optional) canvas sealer. Mirror plates and screws.
Step 1 - Preperation
Fig.2.1 shows a corner of a support with it's edges smoothed using sandpaper. This prevents the canvas tearing when stretching. Don't over smooth the corners.
Place your uncut canvas on a clean and flat surface larger than the support. Position one corner of the support far enough from one corner of the canvas so that when the canvas is folded around the support there is enough to get a good grip. Trim any excess material from the opposite sides so the support is roughly central (see fig.2.2).
Step 2 - Stretching
How tight you stretch the canvas is up to you. Bear in mind that tension is lost after priming, and whether or not you like to build up heavy layers of paint.
Begin by folding one of the shorter edges around the support and fire a couple of staples in the middle of said side. Then take the opposite edge and literally stretch, by pulling it around the support. Hold it there and fire a couple of staples in the middle. Do the same for the remaining sides and you'll have something similiar to fig.2.3
Here comes the slightly tricky part - the corners. I go with the folding approach, rather than making cuts (thus introducing weakness). This does though cause slight bumps at each corner. Decide which sides you want the bumps to be on. Stretch the adjaecent side as shown in fig.2.4. Play around with the folds until they're square when folded down tight. Maintain the tension and staple it, remember the orders of the folds, and the position of the first staple, so the other corners match. Repeat the process for the three remaining corners.
To finish stretching, work around the canvas in stretching and stapling at midpoints between staples already in place.
Continue until the canvas is uniformly stretched. You can feel by running your fingers over the surface, if you can feel ridges of tension in it, it may need more staples.
Tidy dangling threads. You might want to fix the canvas to your studio wall before priming. If you're going to be painting the entire canvas, sides and all, then prime the back edges before fixing it to the wall. Screw two mirror plates on the back, and for very large canvases, use four.
Step 3 - Priming
Here we are using acrylic primer. Acrylic primer can be used for acrylic or oil paintings. Canvas sealer can also be user before priming if you're planning on using oil paints. When using oil paints it is important the oils do not soak into the canvas otherwise they will cause rot.
It is better to use thinned primer and many coats rather than thick primer and few coats. This is especially important if you're going to be using oil paints. I use roughly four parts primer to one part water.
Alternate the coats between horizontal-only brush marks and vertical-only brush marks. You can optionally lessen the roughness of the canvas texture by lightly sanding once each coat is dry. I generally give four coats for acrylics, but for oils I'd use twice as many again.
Occasionally the canvas of a completed painting might loose it's tension and the surface warps. Dampen the back of the canvas with water and as it dries, the tension will be restored.