I was mainly running off of this post by Matt in another thread

Quote Originally Posted by M. A. C. Newsome View Post
To answer your specific question, there are a few different ways to make a fly plaid. Here is one simple, typical method. The deminsions are roughly 54" square. Most tartan cloth now comes 54" wide. Get a yard and a half of it, and that's 54" x 54". Fringe all four sides. How long of a fringe is up to your own personal taste. You don't want it to be very short, but if it's too long it will just tangle. I say a couple of inches is fine. If you want the fringe to be four or five inches, you might want to purle it.

Then take one corner and sew in some pleats running several inches down, and press them. This is the part that will get pinned at the shoulder. The rest hangs free at the back.
The ones Rex posted appear not to be pleated, so much as gathered. They look what I've imagined, though I've admittedly not studied them much. Is there pleating we're not seeing, or is it just gathered?