Processed the mustard seeds. The procedure, according to the internets: pick mostly-dry seed pods/branches (before the birds eat them all); let dry completely; thresh by crushing/mashing/stomping on them in/over something to catch the seeds; sift out the chaff; winnow any remaining chaff (preferably outside), either by gently blowing across it (for small amounts) or by pouring seeds and chaff from one container to another. I did it indoors, and about had a asthma attack from all the dust.
Mashing threshing with a potato masher, in a colander, set over our big stainless steel bowl:
Before winnowing:
Mustard seeds! I ended up with about a 1/4 cup.