How to install custom plugins in Discourse? Beginners Guide

I have always used shared hosting for my Wordpress sites and I have never came across cloud hosting before. It is really easy uploading files on Wordpress sites using FTP clients like FileZilla
I want to know how can I install official discourse plugins in my forum which is setup on digital ocean.
I need step by step guide. I have never came across the word DOCKER ever in my life so kindly be specific in explaining this

Consider me as a beginner who knows only stuff regarding shared hosting and Wordpress installation and who is new to Cloud hosting and DOcKER stuff

Are official discourse plugins not auto installed in upgrade process or we need to add them ourselves as per requirement. Take an example of the plugin bbcode color tag and guide me how can I add it to my forum ask.mybloggertricks.com

Already exists in the howto category here and linked from the plugins page in Discourse itself.