Below are a few guides which will help you get started with Rocket.Chat App CLI to create and publish your own applications.
Rocket.Chat provides an Apps Engine CLI which helps you easily create, deploy, and publish your own extensions for it. This guide will help you get started with the CLI to start creating your own apps.
We had a webinar some time ago on how to get started with Rocket.Chat Apps development. It is a bit outdated, but can get you through the basics. And updated guide can be found at Creating an App.