How do I create a patch?

[Jim Cheetham]

I've found a package with a small problem, that can be fixed with a 
change to one line in one of the config files ...

I'd like to create "a patch" to offer back to Ubuntu ... but I don't 
know how to create one.

I know it involves using 'diff' :-) and I guess I should be changing the 
source package, or perhaps both ... I've downloaded them both, and I've 
made a copy with my change in ... but how do I create the patch correctly?

-jim