Update: This post (from several days ago) has been submitted to the Simple Lives Thusday blog hop: http://gnowfglins.com/2011/02/17/simple-lives-thursday-31/
Grind 3 tablespoons of whole mustard seeds, leaving them mostly whole. You may use a grinder or mortar and pestle.
3 tablespoons mustard powder
1/2 teaspoon salt
3 tablespoons cold water
1 tablespoon cold cider vinegar
Store in a glass jar in the fridge (or a cool area) for a full day before sampling. Really, you don't want to taste it that first day; you just might throw it out!
After the first day, I decide if I like the consistency. If I'd like a smoother mustard, I blend it to the desired texture. Once you know the consistency you prefer, you can make the adjustment on the front end (during the grinding) and eliminate the need for a blender.