Mustard is one of the easiest things I have ever made. Although it is easy to come by, making it yourself is the key to having those crazy customized mustards for a fragment of what they usually cost. This recipe is a guideline, but feel free to add herbs (rosemary, tarragon, basil..) or spices (turmeric, coriander, allspice berries..) or to swap the liquids, just try to respect the ratio of mustard seeds to liquid.
Making it yourself also allows you to control how strong or mellow it is. But most of all, it is a fun little experiment.