I can't seem to find any instructions for square hemstitching. I would really like to give it a try

Hi Carla, I’m not sure what you mean by square hemstitching. Have you done a Google search? There are a couple of books you might be able to find in the library - one that I like is Finishing Techniques for the Handweaver by Virginia M. West.

I saw it in a post somewhere down the rabbit hole. It has little squares on one side of the fabric and Xs on the other side.

Was it Italian Hemstitching? Jane teaches it in the 4 or 5 season with the twill bound weave gamp if I recall correctly