faire la confiture = make THE jam (as in "the jam that I mentioned before")
faire de la confiture = make SOME jam.
There is no "make jam" (with no article) in French. You have to pick between "the" (le, la, les) and "some" (du, de la, des). In your example, "some jam" (de la confiture) works best because I don't know yet which jam you are talking about.