Open educational resources are becoming increasingly important in higher education, and are a valuable resource for instructors who want to customize course content while saving their students money. We designed, revised, and assessed an open educational resource for our Principles of Microbiology course. Student feedback was used to guide the revisions, which took place over the course of several semesters. Student survey responses to lab manual content were very positive, and students overwhelmingly favored a no-cost online manual over one that is commercially published. The process we used to develop this lab manual serves as an example for others who might want to develop their own customized materials for their courses.