AsyncAPI How-To
AsyncAPI 2.x is the latest version of the AsyncAPI Specification, an open-source format for describing and documenting event-driven APIs.
The AsyncAPI website provides tutorials and a full specification for AsyncAPI:
Coming from OpenAPI - comparing/contrasting OpenAPI and AsyncAPI
If you are using OpenAPI 2.0 (Swagger 2.0), see this tutorial instead. If you are using OpenAPI 3.0, see this tutorial instead.