The Okapi Guide and Reference provides the necessary background understanding.
Then see the guide to Running a local FOLIO system for guidance to establish your local FOLIO work environment.
The Commence a module - structure and configuration guide explains a consistent layout for a back-end module.
The RAML Module Builder (RMB) framework, is a special FOLIO module that abstracts much functionality and enables the developer to focus on implementing business functions. Define the APIs and objects in RAML files and schema files, then the RMB generates code and provides tools to help implement the module. (Note that at this stage of the FOLIO project, only this Java-based framework is available.)
The Primer for RAML and JSON Schema provides some guidance.