Since this thread is alive again I thought I would share some tips I developed during the process of doing a couple of dozen of these.
First, for aligning the frame with the picture. I print the images with mirror-image edges, which almost always makes the exact corners easy to spot on the image side of the canvas. I take a very fine sewing needle and poke it thru from the front at the exact corner and then place an ink dot on the back of the canvas just *inside* where the needle emerges. Then I make light pencil lines between these dots. (Doing the lines just a little inside the true corners keeps the lines visible as you position the bar assembly.)
Second, I don't crop my pictures to any specific aspect ratio, so 2 of the framing bars are never exactly the right length, and depending on what I'm after, sometimes 4 are not the right length. Adjusting the bars is best done in the middle, with a glued splice, not at the ends, where you have to cut at a angle and recreate the brace slot.
Third, after trying both ways, I am now leaving enough canvas to fold over the back. Its a neater and more secure job. However, I'm using glue instead of staples on the back for fear of splitting the relatively hard, thin wood of the frames.
Overall, I think I have to salute whoever did the original invention of this system. It's neat & it's nicely engineered.