Wow they are already popping up! Woo nice! I am so glad they are growing happily!

For mustard seeds once they start to bolt, just let them bolt and once the seed stalks and pods have dried up, clip them off. Sometimes they pods start to split so you know they're ready to harvest if that happens. Then put them in a bin or on a sheet and stomp on them and keep stomping and dancing until you've crushed them good. The seeds will be at the bottom, you can pull off the bulky stuff and toss it, then use a fan or the wind to winnow the chaff of the seeds.. I use a two bin method in front of my fan. You can see an example as I winnow peas in the video at the bottom of this post: https://steemit.com/homesteading/@sagescrub/anouas1j

Hope it helps you! Of course theres tons of other info online, but this is the easiest way I found, and very enjoyable and relaxing to stomp and winnow :)