First of all there are generally felt to be very few instances when a fly plaid is needed or considered appropriate-----your wedding, high end black tie events or potentially even more formal situations, like being received by the queen. So although many have fly plaids, outside of Scotland there are not really a whole lot of need for them, so most times you see them in what many would consider "inappropriate" uses and situations.

If you decide you desire to wear one, most have a pleated segment about 8-10 inches in from one corner. Pull that corner through the left epaulet of your PC or more formal jacket from back to front so the pleated part is centered under the epaulet and the small corner hangs down toward your left breast pocket. Then attach your plaid brooch to that corner below your left epaulet. I do not pin the plaid to my jacket fabric , but rather use it as a counterweight to the fly plaid weight hanging down your left back. Some do pin it through the jacket fabric but run the risk of jacket front damage should the plaid be inadvertently yanked strongly. Still others will put the plaid brooch on the epaulet and pin through the epaulet and the plaid.

Others should chime in with further info and questions and comments.