Grind the whole mustard seeds in a spice grinder or by hand with a mortar and pestle. You can also "grind" them in a plastic zip-top bag and a rolling pin. Just crush and grind the seeds until they are very small pieces but not fully fine dust.
Add the ground-up seeds to a bowl and then add the mustard powder. Next, add the other dry ingredients and stir to combine well.
Pour in the beer and mustard and stir. Allow this mixture to sit for 10 minutes and then add the vinegar. Stir again until everything is combined.
The mustard will be pretty liquidy at this point but add it to a jar or sealed container and store in the refrigerator for at least 24 hours and it will thicken up. Mustard can be safely kept in the fridge for many months or up to a year.