Allow your mint plants to flower.
Monitor your mint plants. When blooms begin to turn brown and dry, you are ready to harvest mint seed.
Cut the flower heads from the plant and place them in a paper bag.
Over a bowl, crush each flower head with your hands and allow the seeds to fall into a bowl.
Remove any chaff or debris from the seeds.
Spread the seeds in a pie pan and allow them to dry for a couple days.
Place the seeds in an envelope and keep the envelope in a cool, dry location until spring.