https://fontawesome.com/how-to-use/on-the-web/setup/hosting-font-awesome-yourself
the fontawesome-free-5.11.2-web.zip file is structured as follow :
css
|_ all.css
|_ .......
js
less
metadata
scss
sprites
svgs
webfonts
to implement self hosting, all.css and the entire webfonts dir are to be injected in the static assets folder :
assets
|_ scss
|_ main.scss
|_ all.css
|_ webfonts
Then modify the <head></head> section of _includes/head.html so that it contains :
<link rel="stylesheet" href="{{ "/assets/css/all.css" | relative_url }}">
other option is to use scss :
https://fontawesome.com/how-to-use/on-the-web/using-with/sass