Assuming you installed Ruby and Bundler first. Check the prerequisite section here if you haven’t already

Go to the directory you want to keep the site in. For example, I will be putting in docs. Now create jekyll site with jekyll new . This created a jekyll site in the current directory.

Open the Gemfile that jekyll created.
Comment out the line that has gem "jekyll". So that it looks like #gem "jekyll"
Specify just-the-docs theme by adding this line in Gemfile: gem "just-the-docs".
Close the Gemfile.
Open _config.yml and set theme in the line: theme:minima to theme:just-the-docs
execute: bundle or bundle update
Optionally, make other changes in the _config.yml file for values like title, email, description, url, baseurl, twitter_username, github_username etc.
Close the _config.yml file.
Test the site with bundle exec jekyll serve
Visit http://127.0.0.1:4000/docs/

Acknowledgements:
https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/creating-a-github-pages-site-with-jekyll
https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/testing-your-github-pages-site-locally-with-jekyll
https://github.com/pmarsceill/just-the-docs