Finger press to form a crease. Cut the bottom piece at the crease. Do not cut the top piece yet.
Open up the bottom piece,
and rotate it to the right, so that it's left edge lines up with the fold of the top piece (arrow).
Pin the two together, and mark a line from corner to corner, if necessary.
Sew on the line, and check the fit before trimming.
Trim the diagonal seam to 1/4", and finger press open. Continue attaching the binding to the quilt.
So there you have it! A quick, easy, binding join.
Hope you give this a try--it really is super easy!