I don’t, usually, but once in a while I get the urge.
Like a lot of things, it helps if you make it frequently to learn what your taste might be. There are two things that make this frustrating enough that I usually pass and buy. Item one is quantity – I don’t eat mustard on EVERYTHING, and making a lot of batches means having a lot of waste. Item two is age. I’ll get there in just a moment.
At its heart, prepared mustard is basically a paste made of ground mustard seeds, liquid, and maybe a bit of salt and other spice. Except there are a couple of peculiarities of which you must beware.
Peculiarity one has to do with the enzyme in mustard. Mustard, like several plants, has a self-defense mechanism. Break a cell wall and you release an enzyme. The enzyme is inert unless/until it meets water. At that point you get a reaction. HOT, HOTHOTHOT owowowow… And yes, it’s not just spice heat. It’s a mildly exothermic reaction as well.
Thus enters “age”. The enzyme exhausts itself over time. In the process of aging it interacts with the other flavors of your paste, with everything (mostly) mellowing. In other words you can’t just mix and eat and get the flavor you want. Well, you can, but you can’t get the fuller flavors without waiting — a few days to a week.
One last point before I move on. I do not know why it does so, but the enzyme pretty much stops changing when it gets cold. So if you’ve made a mouth-searing mix that’s exactly what you want, put it in the refrigerator. On the other hand if you want it to mellow and mix, keep it in a room-temperature location till it reaches your point.
But you can see why it’s hard to get a ‘perfect’ mix when you’re only an occasional eater, right?
Still, I encourage experimentation. For that reason, some basics.
Basic one: powdered mustard works. Yes, grinding your own mustard seed is “better”, but if you’re still in early experimentation save some money and effort.
Basic two: mix to the consistency you want up front. The aging won’t thicken it significantly.
Basic three (and obvious): different liquids give different results. Chinese mustard uses water. White vinegar moves you toward French’s mustard. Grey Poupon uses white wine. Do not fear beer, apple cider, or any other liquid provided you like the taste. (I made a mustard using coffee, once. Interesting, but it was chinese mustard with coffee overtones — not my usual preference.)
Oh – if it’s not yellow enough for you (and seed isn’t), use some turmeric. Or saffron, if you’ve money to throw around that way. (It’d be a subtle taste under everything else – not really worth it imo.)
As for additional spices… sugar or honey? ginger? cumin?
(That reminds me. Soak some mustard seeds and grind them and you get those mustard grits you find in some ‘fancy’ mustards.)
I invite you to give it a try, but don’t feel you’re missing some epicurean miracle by saying no. Just remember – the taste will mellow and merge if you leave it on the counter for a few days, and when it’s the way you want it put it into the refrigerator for a while. Beyond that: mustard, liquid, and whatever you want to try.