Squeeze the marigold flower head gently after it has bloomed and its petals have wilted and fallen away. It's ready for harvesting when it feels dry and papery. If it still feels soft and supple, wait a few more days before testing it again.
Snip off the flower head and 1 or 2 inches of its stem with pruning shears.
Lay the flower heads on a slightly absorbent surface. The University of Illinois recommends newspaper. Place the newspaper in a cool, dry location and let the flower heads air dry for two weeks to ensure they're properly dehydrated.
Crumble the flower heads between your fingers. The papery exterior will give way to reveal thin, dark brown or black seeds.
Pick the seeds out and place them in a paper bag. Store the paper bag in a dark, cool and dry location.