Anyone that has stretched canvas has struggled with making folds and cutting the corners cleanly. If you print your own canvas and are permitted to cut the excess canvas then you may find this useful. The following link is to a video of the way one customer is making folds and cutting out the excess.
Making Canvas Folds and Cutting Excess Canvas
Pictured below is yet another method for removing canvas. This technique is quick and very easy and reduces the build-up of canvas in the corners.
The first cut is toward the stretcher bar along the edge:
Cutting Canvas
The second cut is along the stretcher bar inward.
Cutting Canvas
Now make your folds as you usually would.