Guiding physical assembly tasks with AR-based instructions offers efficient, in-situ support to operators in industrial maintenance and ready-to-assemble products. However, the literature is lacking in comprehensive and robust design guidelines for AR-based assembly instructions. The core methodological challenge in developing such guidelines for cutting-edge technology, like AR, is the scarcity of information resources (e.g., real users, experienced designers, a standard feature set). We establish a novel, systematic mixed-methods pipeline that overcomes these limitations. The pipeline is comprised of: (1) identifying design patterns by analyzing publicly-accessible online videos on AR-based assembly instructions, (2) leveraging academic literature to codify the design patterns into preliminary guidelines via critical reasoning, and (3) refining and validating the guidelines according to experts’ ratings, critiques, and evaluations. As a result, we present a robust and comprehensive set of guidelines for AR-based assembly instructions. Further, we reflect on our methodological framework, which is grounded in experts' perspectives, practices, and processes.