Becoming a developmental book editor involves acquiring a mix of education and experience. To begin, if you don't already hold a bachelor's degree in English, journalism, communications, or a related field, it would be beneficial to pursue one. During this degree, you should focus on reading a wide variety of books, plus courses in grammar, composition, and editing.
You should also look for internship opportunities that involve editing manuscripts. Doing this will give you a taste of what the job of developmental book editor really entails, as well as build up your portfolio. It would also be beneficial to join a professional association, such as the Editorial Freelancers Association or the Society for Editors and Proofreaders, in order to network and stay up to date on the latest industry trends.
Last but not least, you'll need to develop a strong understanding of developmental editing, which involves more than basic copyediting. Possible skills include the ability to assess structure and narrative, to work as part of a team with writers and other editors, as well as having a good eye for themes and plot deficiencies. With these combined, you'll be well on your way to becoming a successful developmental book editor.