For mild mustard, combine the water and vinegar and bring to a boil, then mix in the dry ingredients.
Using boiling hot vinegar and water will significantly reduce the nose hit from the mustard powder.
Use cold liquids for hotter mustard. For variations, try substituting a dry white wine or lemon juice in place of the water.
Mustard seeds/powder can be any type: mild, hot, yellow, or brown. Add whole or crushed mustard seeds after completing the basic recipe. They adsorb a lot of liquid. If extra liquid is needed, consider using vinegar first.
You can experiment with numerous other spices and flavors.
The most popular additions are turmeric and paprika.