Over three years, our research team has designed various learning supports for promoting content knowledge and solving game levels. In this case study, we examined the optimal design and the evaluation of learning support videos for a physics educational game. Often studies focus on investigating the effects of research-based principles without a systematic examination of the design and development processes. Thus, comprehensive design descriptions and recommendations for developing effective in-game learning supports are scarce in the literature. This study comprises two stages: design and evaluation. In the design stage, we collaborated with two physics experts to design and iteratively revise 18 learning support videos. We applied the First Principles of Instruction (Merrill, 2002) to create instructional strategies and multimedia learning principles (Mayer, 2017) to develop the videos and help learners engage in cognitive processing. In the evaluation stage, we presented the videos to 14 students to gather feedback on their perceptions and, in the following year, examined the effectiveness of the final videos with 263 students. Results revealed that, among all supports, the videos were the only support that significantly predicted posttest scores and game levels completed and viewing patterns did not affect game enjoyment. We conclude with a discussion of our experiences and recommendations to contribute to the foundation of designing in-game learning supports. [For the corresponding grantee submission, see ED612302.]