Watch for the moonflowers to fade in late summer or early autumn. Eventually, the faded blooms will be replaced with dark purple seed pods.
When the seed pods turn brown and begin to crack open, pick them off the vine. Harvest them soon, because if you leave them too long, they will explode and the seeds will be lost. Put the seed pods in a bowl until you're ready to extract the seeds.
Open the seed pods with your fingers and extract the small, beige moonflower seeds. Put the seeds in an envelope or a small paper sack and leave them in a cool, dry place until spring.