Pick the whole seed heads of fennel, cumin, caraway, coriander, marigolds and sunflowers while still slightly under ripe. Cut them off with a length of stalk still attached and suspend them head down. Wrap a muslin cloth or paper bag around the seed head. When the seeds are dry, they will fall from the seedpod into the bag.
Carefully remove the bag from time to time and sift out the seeds.