Design guidelines serve as an important conceptual tool to guide designers of interactive applications with well-established principles and heuristics. Consulting domain experts is a common way to develop guidelines. However, experts are often not easily accessible, and their time can be expensive. This problem poses challenges in developing comprehensive and practical guidelines. We propose a new guideline development method that uses online public videos as the basis for capturing diverse patterns of design goals and interaction primitives. In a case study focusing on AR-based assembly instructions, we apply our novel Identify-Rationalize pipeline, which distills design patterns from videos featuring AR-based assembly instructions (N=146) into a set of guidelines that cover a wide range of design considerations. The evaluation conducted with 16 AR designers indicated that the pipeline is useful for generating comprehensive guidelines. We conclude by discussing the transferability and practicality of our method.