On collecting mustard seeds . . .
The seeds are small. You might want to do your seed collecting over a jellyroll pan. This photo shows the seed pod with one of its two sides peeled off and the center where the seeds are held. There are seeds on both sides of the center piece. It is easy to break the sides off at the stem end and let the seeds fall into the pan.
You can just crush up the whole plant but you will then have to separate the chaff from the seed. The seeds were too large to go through my sieve so this time I am doing the pods individually.