At its simplest, it is basically finely ground mustard seeds mixed with vinegar. (greenevi.com)
Another basic recipe for making smoother mustard is to grind mustard seeds into a flour, then mix it with a liquid until you achieve the consistency you prefer. (chilipeppermadness.com)
Add one tablespoon chopped cilantro, one teaspoon of minced serrano chile, one teaspoon of ground mustard seed, one tablespoon of lemon juice, 1/4 teaspoon of salt, and 1/4 teaspoon of olive oil. (jerryjamesstone.com)