This article guides the reader through designing a web-based RESTful API with the OpenAPI Specification, starting from setting up the editor and adding information about the API to defining paths, methods, parameters, schemas, responses, and collaborating on the design. The author emphasizes the importance of considering consumers in the design process, using principles such as usability, findability, value, credibility, usefulness, and desirability, and providing clear documentation for the API. The reader learns how to create a simple API specification with two resources, Principles and Best Practices, and add GET, POST, and DELETE methods to it. The tutorial concludes by discussing collaboration tools and exporting the definition as a YAML file.