https://developer.atlassian.com/platform/forge/

In order to create a custom forge app, you have to download forge using npm

How? Run in your terminal 

npm install -g @forge/cli
## When it's done, you can test the installation with forge --version
## Output should be something like 6.4.1

Don't you have npm? Mmm..maybe you don't have node too. So... 

From NodeJs official website, install Node on you OS (it will install npm as well), then run in your terminal

node -v
## Output should be something like v15.2.3
npm -v
## Output should be something like 6.14.3

Don't forget to install forge (see above), after node installation!

Login to your instance

How? Run in your terminal (choose a folder as workspace for your apps)

forge login
## Enter your email: your email used to have access to your cloud instance
## Enter your api token: your api token related to your email used to have access to your cloud instance
## Allow Forge to collect CLI usage and error reporting information? YES

If you hadn't create a api token, you should create it and use it to complete login process.

Creation of you first forge app

How? Run in your terminal (choose a folder as workspace for your apps)

forge create

## Enter a name for your app (up to 50 characters). For example, hello-world-app.
## Select the UI kit category.
## Select the jira-issue-panel template.
## Change to the app subdirectory to see the app files:

cd hello-world-app


Deploy of you first forge app

How? Run in your terminal (choose your app folder)

## Navigate to the app's top-level directory and deploy your app by running:
forge deploy
## Install your app by running:
forge install
## Type the hostname of you cloud instance when the prompt will ask you