Take the loose end of your binding and fold it away from your quilt, so that it follows the little angle you have just added, and the raw edge of the loose end is even with the next edge that you will be sewing the binding onto.
Keeping the little crease you just made, fold the loose end down and into place along the edge to be stitched, raw wedge matched with the edge of your quilt. Begin stitching right at the upper edge.
Your corners will look like this...
Until you whipstitch the loose edge onto the back - you will find you have perfect, neat, square corners - like they just want to be square!
Thanks Kaye Woods, for the great tip!