First of all Font Awesome Extension requires Font Awesome. You need to download it and include into your project.
After include Font Awesome into you project you must download Font Awesome Extension as .zip or tar.gz and include CSS file (You can use .min or normal CSS file) into your project. Take care the structure of directories, Font Awesome Extension has CSS file and folder that contains the fonts at the same level, dont't change this ;) .
After include booth CSS file from Font Awesome and Font Awesome Extension you can invoque the icons, you must use it on element i or span,
be sure to include class fa (From Font Awesome) and include the secondary class from the icon that you want as fae-icon (from Font Awesome Extension) or fa-icon (from Font Awesome).
< i class="fa fae-tooth">< /i>
You can combine Font Awesome classes to Font Awesome Extension to the icon that you want use