My painting has started when I begin to stretch my canvas; I'm getting in touch with its size and shape.
As for the "how to"; you will need to go to an art supply store and get four stretcher pieces, perhaps start small with two 24" and two 18" pieces. Close one eye and hold each piece up to look along it to see it's fairly straight. Often they get bent or warped. Then buy canvas that is at least one and a half inches wider than your stretcher all the way around. I have a roll of 10 oz canvas that is five foot high. Canvas pliers are also useful if you want to stretch larger pieces.
If you are going to do much of this you will want a trip to your local hardware store as well, for a JT21 stapler and 1/4" staples. They are light and easy to use.
Now for the fun part, back at your studio lay out your canvas on a table and push your stretcher pieces together at the corners. Gently use a hammer if you need to. Use a set square (or a sheet of paper), to check your corners are "square" (actually at 90 degrees to each other). With the stretcher on the canvas make about a one inch cut where you need to reduce the size of the canvas, then tear, yes TEAR the canvas in two at that point. Smells good doesn't it?
Now you can put in a couple of staples in the middle of a long side about 2-3" apart; go round to the other side of the stretcher and make sure everything is still square; pull the canvas taught in the middle and put in 2 staples there as well. Pulling away from you and towards the corner put in 1 or 2 more. Keep alternating sides till two edges are stapled and the canvas weft looks straight on the stretcher. Then do the same with the shorter sides. The corners are just like hospital bed sheets and need neatly folded away and stapled flat. Hopefully my photo here will help you see how to do that.
Next I soak the canvas and its edges as evenly as possible using a sponge and warm water. This will shrink the canvas and should make it as tight as a drum. Leave it lying flat to dry. Tip it to the light to see you have all of it wet, because of the sizing in the canvas the water will want to just "sit" on the top.
This technique is good if you want to do some stain painting first a la Helen Frankenthaller; and because you shrank the canvas first you can leave bits of raw canvas if you want. I enjoy working In layers, on several paintings on the same day, watching them dry (they tend to lighten), and feeding them paint or water, whatever I feel they need. Next week I'll tell you about some other acrylic techniques you can use.