In the repository settings, navigate to GitHub pages. Select a branch to build the website from, a theme if desired, and a custom domain if desired. Save the settings.
Go to the _posts/ directory and create a new file with the format: YYYY-MM-DD-title-of-post.md
Post titles must be in this format to work correctly!
Format the contents of the post like so:
---
layout: post
title: title of your blog post
---
Then, simply write your post here, using any desired formatting.
References:
Writing Blog Posts: https://jekyllrb.com/docs/posts/
Markdown Syntax: https://www.markdownguide.org/basic-syntax/