When I first started to weave, I took a nine month long class called Weaver's Boot Camp. This particular class assignment was to design a two-block lace pattern and weave 4 place-mats. Woven lace is a loom generated fabric that has holes in it. I chose Swedish Lace with lattice hem-stitching and made table napkins, instead of place-mats because my yarn was pretty fine. I have included my finishing technique for mitering corners with these step-by-step instructions and photos. Feel free to borrow, but please include a link back to this post. Thank you.
Wet finish the piece and steam press before starting so the weaving yarn is set. Turn edge in 1/4" and press.
A chalk line is marked diagonally on the corner from hem crease line to crease line.
If you are trying this without the hem-stitching, you will need to determine how deep your hem will be. From the finished photo above, you can see that mine is a fairly deep hem, folding up to the first edge of the first hem-stitching row. You may want to chalk mark that also.
Fold corner, hems and right sides together. Stitch on chalk line, from hem fold (not the edge) to diagonal fold. Use a fairly small stitch. Back stitch both ends.
Note that the stitching only goes to the hem fold, not the edge. Trim the seam to 1/4". The hem is not turned down while trimming.
At this point, if your fabric ravels easily, you could carefully treat the cut edge with Dritz Fray Check. It is a permanent treatment, and is a bit stiff, so make sure it doesn't soak through to any area that will be on the outside.
Turn the pressed 1/4" edge hem on both sides.
Dampen corner and finger press diagonal seam open. Raveling can be a problem, so use care.
Carefully turn the corner, gently working the point out with a blunt object. I used a medium sized crochet hook.
Fold hem and pin in place. Repeat for the other 3 corners.
Hand or machine stitch hem. I like to finish mine on the sewing machine. I set the zigzag so it will catch one thread of the hem...
and then one stitch into the hemstitching space. Adjust the stitch length to try and avoid hitting the hem-stitching threads. If your thread matches the fabric, you will be unable to tell that it is machine stitched unless you examine it very closely.
If you are doing mitered hems without the hemstitching, the machine stitching can be a straight stitch.
Done and ready for pressing!