Follow the instructions at GitHub.
Add an index.md (or index.html) file to the repo.
Paste https://username.github.io into your browser.
That’s it.
Troubleshooting: First time I did this, it worked without any problems. When I set up one for my son, we run into troubles. It tried everything described at My new Github page isn’t showing up. What helped was to
_config.yaml file (if one wants to get rid of the theme).2 and 3 cancel each other out. I have no idea why it was necessary to insert these steps.
Fancy stuff: Currently, I am not using anything else than markdown written in the editor visual studio code, which has a good git integration. (This is the first time that I am convinced by a microsoft product.)
In case I need something more fancy in the future I intend to collect here some links here that may be good starting points for further developments.