As the petals fall off, the center florets dry up and the seed kernels begin to swell in the disks.  To protect the seeds from birds and rodents, cover the flower head with a mesh onion bag or paper bag. Or, when the seed heads start to turn brown and the ripened seeds develop a hard shell, they can be cut with 2 inches of the stem and hung to dry in a ventilated place such as a garage or attic.

When the seeds are completely dry,  remove them by hand or by rubbing them over wire mesh into a basket. Store seeds in a tight closed container.