Your fly plaid has one portion of the material that is gathered and sewn together. Find this section and put the fabric through the epaulette as shown in the picture.

Position the fly plaid so that it looks like this.

Insert the fly plaid pin as seen in the picture here. The pin should go through the fly plaid as well as the jacket material. The pin should be inserted facing down so that the clasp does up at the bottom. If you are using a smaller fly plaid pin, you may want to also reinforce the fabric and pin so that it stays in place. This can be done by pinning the fabric to the jacket with safety pins (underneath the fly plaid).

The finished result.