Follow the install notes.
The theme is free to use, but if you do use it, it would be great to hear from you. Email simon@freytag.org.uk - thanks!
This theme uses {{site.baseurl}} throughout.
This means it works well gith GitHub pages and Jekyll 3.9, where the site is served out of a folder - eg https://sfreytag.github.io/friday-theme/
It also works with Jekyll 4.* and where the site is served out of the root of the domain - eg https://www.freytag.org.uk
Look at _config.yml
and the baseurl
setting.