Gmff, welcome to heck. Well, only if you hate stretching canvas. After a while you either price the dickens out of it or you just do it as the next job.
If this is going to look good, use the thick stretcher bars (1 1/8" thick and about 1-1/2"wide). staples go on the back about 1/2" in from the edge. Neatness counts.
As you are starting with raw primed canvas, you can "pre-crease" the canvas and hone the crease with a bone. This will help with the corners.
When you get within three inches on both sides of a corner, push the center of the canvas corner to the wood corner. Hold this "centered" as you begin to fold the sides till they make a nice joint at the corner. This should looks like "hospital sheet corners" tight and straight.
(I used to do this at Boot Camp an drove the CC nuts.)
Under both sides, you should have a fold running from the face corner towards the middle at a 45 degree. Now fold the one side over the back and staple. Repeat with other side. and the corner will be square and smooth. Not? Oh, that fold is a bump? Grab your hammer and pound it a bit!
A couple of whacks and it will be flat.
This type of corner leaves a paintable corner that is flat to the corner.
And this is the only $$s you will get from this artist or their customer, so charge accordingly. I charge about double what I would normally get for shop rate. If they pay it; they are lazier then they are poor.
It always amazes me that art schools teach painting, but NOT how to stretch your own canvas.
BTW: You can't "Key" this kind of corner.
