Fold quilt diagonally, right sides together. Pin along border seams. As you manipulate the seams, you will feel the seams snuggle up to each other. This is because the seams were pressed in opposite directions. Pin along the drawn stitching line. Stitch line being sure to backstitch at both ends of the line.
Open and check miter. If there is a pucker at the corner, or if your corner is not square, re-do the miter.
Once you are happy with the miter, re-fold with right sides together. Trim seam 1/4” from the stitched line.