When Zill introduces a new method (such as the derivation of the integrating factor formula), close the book and try to derive it yourself on a blank sheet of paper. Understanding why a method works prevents you from panicking when a problem deviates from standard template examples. To help you get the most out of your studies, tell me: What or chapter are you currently stuck on?

If you are studying independently and cannot afford a commercial textbook, consider checking out legal open-source alternatives like Notes on Diffy Qs by Jiří Lebl or OpenStax calculus supplements, which mirror the structural flow found in Zill's outline.