It's really important to measure across the width and mark the fabric for that length. I pin the fabric from marked edge to marked edge making sure I have enough hanging over for the miter. Then I sew from 1/4" inside the edge to 1/4" inside the other edge, back-stitching on the start and end but NOT sewing into the 1/4" seam allowance.
There are different ways how to get the miter marked (or folded and pressed) and sewn for the actual miter.