Allow your red onion plants to form flower spikes in late summer or fall.
Pull onions and cut off the flower heads when they appear brown, dry and crunchy. Transfer them to shoeboxes or other boxes, leaving plenty of space and laying them in a single layer. Leave them in a cool, dark, dry place for two to three weeks.
Shake the seeds into your box or gently rub the seed heads between your palms over the box to release all seeds. If you wind up with some of the chaff, or plant material that surrounds the seeds, that's fine.
Pour all seeds and chaff onto a cookie sheet. Stand somewhere that isn't windy and gently blow on the seeds to separate them from the lighter chaff.
Transfer seeds into plastic zipper bags and store them in the vegetable crisper of your refrigerator until you want to use them to grow more red onions.