Stretching canvas is easy! Stretching primed canvas can be a little tougher than raw canvas, but it will save you time in the long run. Here is a tutorial I made on how to stretch raw canvas. I go into greater detail on how to fold corners, too.
The disadvantage to stretching primed canvas is that you need to get it a little bit tighter; often this is too tough to do if you are just using your fingers. You may find a set of canvas pliers to be helpful.
First, cut a piece of canvas a little bit bigger than your stretcher bars. Since these are thick bars, I've left about 2.5 inches all the way around.
With your canvas primed-side down (and your stretcher bars upside down as well), staple the canvas down in the center of the edge closest to you. Pull the opposite edge tight and then staple that side down. Then staple the two sides.
It's important to staple in the center of each bar and work your way outwards, working on all sides together. This ensures even stretching.
Work your way to the corners, avoiding placing staples a few inches from the corner.
One of my pet peeves is a badly folded corner. A badly stretched canvas and a poorly folded corner is (in my opinion) a sign of poor craftsmanship. I think every part of a painting, from canvas to frame, should be well-crafted, not just the painting itself.
Form the first part of the fold.
Pull the outer part of the fold up and over the first. Keep the seam along the corner. It looks the best!
Pull tight and staple. All done!
The trick when stretching pre-primed canvas is getting it tight enough. If the canvas seems a little loose, try painting a coat of gesso on the front (my preference). It will shrink a bit as it dries, and can help tighten the canvas. Misting the back with water from a spray bottle can help too, if you don't want to gesso the front. Wooden stretcher keys can also be tapped into the corners to pull the canvas tighter.
Once again, you can reference this post for a much more in-depth look at how to fold corners.