In addition to the v2 Spec, I used the online Swagger Editor and followed these two tutorials to get an actual feel for what the Spec requires:
For the v3 OpenAPI Spec, the following resource looks promising as well:
- [Swagger Editor](https://editor.swagger.io/)
- [Writing OpenAPI (Swagger) Specification Tutorial - Part 2 - The Basics | API Handyman](https://apihandyman.io/writing-openapi-swagger-specification-tutorial-part-2-the-basics/)
- [Create Your First OpenAPI Definition with Swagger Editor | BlazeMeter](https://www.blazemeter.com/blog/create-your-first-openapi-definition-with-swagger-editor)
For the v3 OpenAPI Spec, the following resource looks promising as well:
- [Step 1: The openapi object (OpenAPI tutorial) | Documenting REST APIs](https://idratherbewriting.com/learnapidoc/pubapis_openapi_step1_openapi_object.html)