Jenny Doan's method works, but I can make it better. If you press a triangle into the beginning of the binding, that will show you where to cut off the tail. When I start putting on the binding, I fold down the top edge so that all raw edges are lined up on the edge of the quilt. Leaving a tail, sew on the binding. (I like how Jenny turns corners. I've been doing it that way for a few years now.) When you come to the finishing end of the binding, you will find that the ending tail can be placed over the beginning tail, and should be cut off at the base of the triangle. That is exactly the width of the binding. That works no matter how wide your binding is. No 2 1/2 inch to worry about.
Hold the ends RST the way you sewed all the other seams in your binding leaving no "plus sign". Sew those two ends together on the diagonal, following the line you pressed in at the beginning, but leave ears.
One more thing I wish Jenny would do is: Open the seam after it is sewn. We're trying to reduce bulk by sewing all these seams on the diagonal, so opening the seam helps a lot, too.