Question: How Do You Make Mustard Out Of Mustard Powder?
By: Danny Wacker
Published: January 21, 2010

I've got some Coleman's dry mustard powder, and a hot dog. 

How do I put mustard on my hot dog?

Answers:
Helen Pitlick

Mix equal parts mustard powder and water or vinegar, then spread it on your hot dog! You could use wine or beer as the liquid, but water and vinegar are easiest. It's not going to be gourmet, but it'll do the trick.
Colin Saunders

This is a test.
Cynthia Bertelsen

Probably too late and going a little further than your question meant to, but I wrote a post on my blog about making mustards from scratch -- you could just substitute mustard powder if you wish:
Spooning Up Mustard, Thick as Tewksbury
http://gherkinstomatoes.com/2008/08/15/a-thumb-in-the-mustard-pot-thick-as-tewksbury/
Cindy