For example, Font Awesome 6.4.2 Free for Web
Download Font Awesome from https://fontawesome.com/download, default version 6 (can select older versions), click [Free for Web]
Unzip, copy fontawesome-free-6.4.2-web\css\all.min.css into .(site root)\assets\css\extended folder. Edit \assets\css\extended\all.min.css, replace all
../webfonts/
with
../../webfonts/
Copy subfolder webfonts of fontawesome-free-6.4.2-web into .(site root)\static