Originally Posted by kwiltkrazy
When you are sewing on your border you create the miter by bringing your binding up at a 90degree angle from the top of your quilt, then you fold it down, to run along the next side of the quilt. I forgot to say that you stop sewing 1/4inch from the end of the quilt, and startup again 1/4 inch from the edge. Gives you wonderful miters when you fold the binding around to the back.
I was referring to when the border is already sewn on, as you describe. Then some people hand sew the 45 deg miter lines as akrogirl said she does.