Creating an OpenAPI document can be overwhelming, particularly in the early stages of API design, due to the evolving nature of understanding the problem space. To streamline this process, a phased approach is recommended, starting with capturing the essentials of the API, such as its purpose and scope, then progressively adding more detailed operational descriptions and metadata as insights and feedback are received. The article outlines a three-phase process using a TODO list API as an example, showing how to iteratively develop an OpenAPI document. Initially, the focus is on establishing the API's info section, which includes metadata such as title, description, and licensing details. The second phase involves detailing high-level operation paths and methods, defining operation summaries, descriptions, and response codes, and assigning tags for better organization. The final phase emphasizes capturing schema components and remaining operation details, with a focus on request and response schemas, security requirements, and the inclusion of examples to enhance understanding. Each phase is designed to keep the document syntactically valid and adaptable to changes, ensuring it remains a valuable resource throughout the API's development and lifecycle.