If you grow cilantro for its leaves and have never let any of it go to seed, you are missing out on Chapter 2 of this herb. Cilantro will flower (with a fragrance only its mother could love, maybe) then form seed pods. After the seeds pods have dried (in situ is recommended) gather them from the garden. Spread out a large sheet of paper and rub the pods off the stems.
This makes a big mess and much of the brittle, dry stems get rubbed off as well. What comes next is a sort of winnowing–gather the pods and stems into a pile. Most of the stem fragments gather at the top and can be lifted off. I do this a few times until only seed pods are left, almost.
I keep picking over them until they are truly stem free and dump the pods out onto a towel. The small, dusty fragments will fall into the terry cloth of the towel and get trapped. I skim off the seeds and do this another time or two on fresh toweling until the towel comes clean.
Store in a dry jar with a tight lid.