Clip zinnia blossoms from stems once the blossoms have begun to droop and fade. Use scissors to cut the bloom from the stem at the base of the blossom.
Place the clipped blossoms in a paper bag. Do not crowd the flower heads in the sack. You should have only enough blossoms in each sack to loosely cover the bottom of the bag.
Fold down the top of the bag and staple shut.
Hang the bag in a dry place, such as a basement, attic, or closet. Leave undisturbed for six to eight weeks.
Shake the bag vigorously. The dried blossoms should rattle around and the seeds detach from the flower head. At this point, you can open the bag and use your fingers to help separate the dried petals and attached seeds from the flower head. Continue to store in the paper bag or transfer to a glass jar or other airtight container.