Massive Open Online Courses (MOOCs) are a primary mode of scaled-up online learning. They have enjoyed great popularity, which grew significantly during the Covid-19 pandemic. Despite the growing demand for MOOCs and the desire of institutions to develop high-quality courses, research indicates that the educational impact of MOOCs is limited. Low instructional design quality has been cited as a culprit for the failure of MOOCs to support learning and skill development, prompting calls for a more thorough design of MOOCs. The aims of our work are to describe the systematic development of guiding principles that informed instructional design decisions for our MOOC on health behaviors and to showcase how these principles were implemented in practice. Through triangulation of various sources of evidence on anticipated learners and the MOOC learning environment, we identified four key aspects we needed to address with our instructional design to facilitate learning. We formulated guiding principles that both addressed these aspects and served as a reference point during instructional design. Our approach to making instructional design decisions can serve as a practical blueprint for other MOOC developers.