https://www.youtube.com/watch?v=j671d6FyemQ
https://www.americastestkitchen.com/recipes/8749-whole-grain-mustard

1/3 C yellow mustard seeds
1/3 C brown mustard seeds
1/2 C cider vinegar
1/4 C water
2 T light brown sugar
1 1/4 t salt

Combine vinegar, water, and mustard seeds in a medium bowl. Cover with
plastic and let stand at room temperature for 8-24 hours.

Put the contents of the bowl, along with salt and brown sugar in a food
processor and process until coarsely ground and thickened (about 1-2 mins).

Portion mustard into two 1 cup jars. Cover and let stand at room temperature
until it has reeached desired spiciness (1-2 days). Once it's in the
refrigerator, the flavor will mature, but it will not become more spicy.
It can be refrigerated for up to 6 months.