Here is a breakdown of how to make a whole grain mustard at home. This is a base recipe that you can use to start your own mustards. Add in other ingredients to make a unique combination.
The flavors will be quite pungent at first, though they will mellow out over time as the mustard ages. Serve it as is, or process it for a thicker consistency. Vary up the liquids and add additional ingredients to make your own unique mustard recipes. Refer to the post above for ideas and suggestions.