J’ai aussi goûté la confiture en promotion. C’est pour faire/préparer de la confiture de fraises.

C'est pour faire de la confiture de fraise

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.