It can be done easily - what I do is fold the corner point of the backing forward to the front corner. That gets you an edge that is 45 degrees angled from the vertical and horizontal sides. Then fold the side straight towards the middle. You'll now have half a miter at the top edge. Do the same with the top edge and the two will meet together with a perfect miter.
Practise with a piece of paper: top left corner folded at a 45 degree angle to the center, then right edge towards the middle and you'll see the angle appearing. Then top folded down to meet the right edge - perfect miter.
Last edited by ArtsyOne; 08-20-2012 at 10:37 AM.