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).