Despite coverage of materials development principles in the applied linguistics literature, principled production of open textbooks has not received attention. To address this gap and demonstrate the interdisciplinary potential of materials development research, the authors drew upon concurrent verbalization and interview data they collected while composing their first coursebook, a freely available open textbook designed for first-year university writing courses that enroll English first-language and second-language learners, to discern how they applied principles. Qualitative content analysis of the data indicated the novice textbook writers focused on learning objectives to negotiate textbook relevancy and currency concerns, tried to create relatable content, and employed varied repetition and layering to build modular chapters. This study illustrates the generalizability of materials development research and principles and offers a view of open textbook authorship for teachers interested in the activity.