I took a run home to my family's place earlier today. I still have some stuff left at their house, a couple of paintings amongst other things. I took the time to fold some corners on one of them. I think it is kind of tricky to get a good result and Ive tried a couple of techniques. The way Ive done it here is to keep all the fabric (instead of cutting it off) and fold it several times. The end result gets very thick, on the other hand I imagine that it is an advantage to keep as much as possible if you need to restretch the canvas in the future.
Compare this to a "cleaner cut" canvas that was stretched already when I bought it: