1. Unbutton your the epaulet on your left shoulder and flip it open. Place plaid face up over your left shoulder starting with the closely stitched end falling in front of you about 6" below your waist, depending on your height..
2. Wrap the portion falling behind you around your back and under your right arm.
3. Continue wrapping across your chest and back up over your left shoulder. (If you are wearing a crossbelt, traditionally the plaid would go over it as the plaid has its origins as a blanket. However, a number of organizations wear the plaid under the crossbelt, and this is accepted also. Crossbelt buckles will often have a hook with which to secure the plaid as it passes under the lower half of the buckle.)
4. The end of the open pleated portion of the plaid should be hanging at the top of your ankle, otherwise consider your target to be one quarter to one third up your lower leg. Adjust the plaid around your body so that the end is properly positioned.
5. Pull on the the closely stitched end to tighten the plaid close to your body so that it is not loose, but not constricting.
6. Button the epaulet over the plaid.
7. Fold the end of the plaid that is hanging in front of you over the left epaulet—slightly to the outside—then down underneath the length of plaid that was already hanging at your back.
8. The stitched end which is now mostly hidden, is slipped under the crossbelt and also under the belt at your waist. (see picture)
9. Pull the pleat of exposed plaid closest to your left arm all the way around your left shoulder to the front. Using a plaid brooch and with the first pleat remaining closed, pin the plaid to the chest, crossing a portion of the plaid where it meets the shoulder. This point should be at least as high as the base of the left epaulet. Done correctly, the the plaid should drop from the brooch and around your shoulder to your back.

Andrew Lenz Phil Gill and others