I think it was Fons & Porter that had a very easy way to do them...don't give up! I am sure there is a tute out there.
The way they did it was to make the border longer than the edge,,,sew them on but leave 1/4" on each corner then fold the 2 border fabrics right sides together and sew from the corner to the angled point...too hard to describe, but it works. Don't give up!