I haven't just been cooking this week, I have also been making a set of place mats and napkins for some dear friends from some old vintage fabrics and buttons that I have sourced from thrift shops and markets. It took me ages to get the hang of mitred corners so here is my version.
If this were your fabric and you were hemming with a double turn over and machine stitching, then these would be your fabric fold lines. I cut a square from each corner just inside the fold line marks. This reduces bulk to stitch through.
Fold the fabric cut edge along the innermost fold line as shown.
Next make the first fold up.
Now stitch along the fabric closer to the inner edge so that it sits well. I find this gives me a neat fray resistant edge and a flat mitred corner without the bulk (and broken needles).