The next generations of science, technology, engineering, and math (STEM) workers are being trained in college and university classrooms by a workforce of instructors who learn pedagogical practice largely on the job. While inclusive instructional practices and their impacts are increasingly well-studied, this training is difficult to instill within the professional development that most STEM professors receive before teaching their students. The Science Teaching Experience Program for Upcoming PhDs (STEP-UP) at the University of Washington was built to prepare future professors for inclusive excellence by guiding them through the literature in education research and providing them a space to practice active and inclusive teaching techniques. This study of STEP-UP uses a design-based approach to understand graduate trainee and undergraduate perceptions of the most salient aspects and outcomes of the program. Our study found that trainees used opportunities to practice inclusive teaching methods with a cohort of their peers, and crucially that these methods were evident in trainee-taught courses through multiple lines of evidence. STEP-UP-trained instructors used inclusive teaching strategies that helped students to feel socioemotionally supported. This study offers a model program that fosters inclusion and equity in undergraduate STEM classrooms through improving teaching professional development for graduate students.