Use the Creative Grids Quick Trim Ruler. Trim the backing 2" larger than the front. Place the ruler with the 90 degree line on the front section in the corner and trim. Repeat for the other 3 corners. Press each side under 1/2". Now match each individual corner up and stitch (that is your 1/4" stitch line). You will now have 4 perfect mitered corners. Top-stitch at the edge of your 'binding' and you are done.