First, melt some margarine in your pot while boiling some elbow pasta in another pot. Once margarine is melted, add some flour and mix together. Allow this to brown for just a wee bit.
Next add in some milk. Now you have a basic "white sauce".
Aaahhh, pasta's done! Oops, I forgot to add salt to the pasta water. Oh well, it's hard to think of everything.
Now throw in some cheddar cheese. The sharper- the better but in this case we just had mild. We'll fix that in just a second. Okay?
Stir cheese in very well. Oh yeah, add some salt and pepper.
Now, don't spaz out on me, okay? Put in about a tablespoon of mustard. If you have dry mustard, then great. If not then just use some prepared. This will give a little punch to your mac and cheese and will give it a sharp flavor that's desired in cheese. It's good...really! Just try it!