This qualitative study examined how seven graduate students became prepared to teach in an unfamiliar online PBL environment through an experiential professional development program. During the 75-hour preparation program, graduate instructors completed readings and practice activities, immersed themselves in the student perspective, and completed learning tasks that mirrored the teaching tasks to be carried out during the live teaching. This enabled them to immediately apply theory into their own contexts while receiving support. Findings indicate that the most impactful learning methods were active experimentation and communication with peers and mentors. Results can inform academic developers who design and facilitate professional development for graduate students.