Charles Ellicott Commentary Exodus 28:37

"And thou shalt put it on a lace of blue, and it shall be upon the mitre; upon the forefront of the mitre it shall be." — Exodus 28:37 (ASV)

You shall put it on a blue lace. —Compare Exodus 39:31, where we read they tied unto it a lace of blue. Probably the two ends of the plate were perforated, and a blue lace or cord passed through the holes and tied to the plate, which was then put in front of the turban and kept in place by the two cords being tied together at the back of the head.