If you're like me and you're tired of store bought mustard that has no heat and next to no flavor, this is one you should make, especially because it takes almost no work. I took the basic recipe from here, but he doesn't give specifics, and I wouldn't suggest using wine in place of the initial water (it's slightly acidic, which would keep it from developing as much heat).
6 T black mustard seeds, crushed or ground depending on the texture you want
1/2 C mustard powder (I use Colman's)
1/2 C ice water
3 T white wine vinegar
2 t salt
Combine the mustard powder, the mustard seeds, and the salt. Add the ice water, mix thoroughly, and allow to sit for 12 minutes. The timing here is important; the water activates the chemicals that give the mustard it's heat, so longer times develop more, but those chemicals are volatile, so if you wait too long it starts to mellow out. 12 minutes seems to give the most heat, and at the end of this time you add the vinegar and mix to set the reactions. Give it a day before using to blend flavors and lose the initial bitterness. As far as storage, don't worry about refrigerating it, pretty much nothing will grow in it.