Usually Square or sometimes rectangular, it's worn 'diagonally'
It's just a loose 'pleating from one corner-which goes over the shoulder (can be threaded through the Epaulet) I found a black military shirt with epaulets, so I don't have to wear a hot jacket in warm weather. The Fly Plaid is then pinned with a Plaid Brooch (or a large brooch of choice) on the chest. Some plaids come with a pleat stitched in to get you started.