Followed the instructions on a fairly old post (2014) by Barry Clark, I finally managed to build/fork a Jekyll engined blog and have it hosted on GitHub. However, I wanted to host it under
https://<GITHUBUSERNAME>.github.io/blog
rather than
https://<GITHUBUSERNAME>.github.io.

On his post, he states that this would introduces some complexity. True, but fairly easy to resolve as well:
Step 1. Edit the _config.yml file
Step 2. Configuring a publishing source

Step 1. Edit the _config.yml file

Assuming you have forked Clark’s Jekyll-now repository, all you have to do is to find _config.yml file, open it, find Baseurl:, add "/repository-name", it should be looking like this:

Baseurl: "/blog" (if blog is your repository name)

Do NOT forget the / before the repository-name.

Step 2. Configuring a publishing source (If you haven’t done so)

  1. Find the Settings option on your GitHub repository.
  2. Find “GitHub Pages”, select ‘gh-pages branch’ under Source drop-down menu

After a few seconds, refresh the page you should be able to see the green banner under “GitHub Pages” says your site is published at https://<GITHUBUSERNAME>.github.io/<YOURPROJECTNAME> this link.

Click the link and VOILÀ! Simple eh?!

No gh-pages branch needed

Accidentally, I created gh-pages branch at first published from this branch (Settings > GitHub Pages > Source) and switched back to master branch, found out that the site still loads perfectly.

Beware in which branch of the repository you are modifying

As a beginner, be very careful of the branch in which modification takes place. If necessary set the branch to default branch under Settings > Branches > Default branch.