This study investigated the case-based method (CBM) instructional-design theory and its application in designing self-directed online instruction. The purpose of this study was to validate and refine the theory for a self-directed online instruction context. Guided by formative research methodology, this study first developed an online tutorial based on 13 design assumptions synthesized from the CBM literature. The researchers then formatively evaluated the online tutorial as a design instance of CBM through two iterations of design, evaluation, and revision. The major findings included: (1) perceived value of various CBM design features, (2) benefits and limitations of applying CBM in the tutorial design, and (3) validation and revision of a set of generic and context-specific CBM design assumptions. These findings extend our understanding of CBM to the context of self-directed online instruction and provide useful insights and practical guidance to inform instructional design practices.