It isn't so much made as grown. Grow the mustard plant and grind up the seeds. This will give you a yellow powder, mix with water to make a paste and that's English mustard. There is a company called Colman's in the UK that will sell you just the powder in a little tin. I've never liked it - far too hot.
Other varieties such as Dijon mustard or German mustard have other ingredients in them - look at the list of ingredients to see exactly what. They're just mixed with the mustard powder. Much milder because the other ingredients, such as wine vinegar, dilute the taste of the mustard seed. German mustard is much the best to go on a hot dog which, after all, is another German idea.
I've gone totally off the point now but what the hell - the best fast food in the world as far as I'm concerned is to be found in the south of Germany, where you can get Bratwurst in a bread roll with a good dollop of German mustard. I've enjoyed this in the food court at Munich railway station - "Bratwurst im Semmel, bitte" and when you can get that, quite rightly McDonald's is relegated to upstairs. I've also had it as a street snack in Tübingen.