With mustard powder one can make a mustard; this is strong stuff, so you may need to warn those used to the sugary splort bottle type of mustard. Various mustard recipes will call for various ratios; if you have a kitchen scale then it's probably easier to start with equal masses of mustard powder and liquid; this produces a thick paste; you may want a thinner texture?
Mix together. A chopstick can be good if the jar is narrow, or mix it in a bowl and then jar the mess. The texture can be adjusted by adding more or less liquid. Trace amounts of various spices (tumeric, black pepper) might be good to add, or maybe a bit of salt if it is not going into the fridge.
The mixture should be left to sit for about 10 minutes to reach full potential. I usually put it in the fridge and use small portions of it on various things.
There are recipes that involve soaking mustard seeds in beer and then grinding up that mash.