I don't know about hankies, but I am a professional veil maker and this is how I miter my corners. For each corner, bring the sides right side together making sure the lace edges match up. You will then have the lace on the left and the folded edge on the right. With the lace tag ends lined up together and extending to the right, use the folded line as a guide and sew straight up using a small straight stitch. Trim the tag ends fairly close to the stitched line. Open the fold. Now go over your seam using a tiny herringbone or zigzag stitch to hold the ends down and to be sure the straight stitching doesn't come undone. Do this for each corner. Hope this helps.