I need help on installing a plugin

Installation
3

Well, you put the plugin url into the app.yml like shown in the link you posted:

hooks:
  after_code:
    - exec:
        cd: $home/plugins
        cmd:
          - sudo -E -u discourse git clone https://github.com/discourse/docker_manager.git
          - sudo -E -u discourse git clone https://github.com/discourse/discourse-spoiler-alert.git

Here in the example, the plugin is discourse spoiler alert.