Using icons in CustomUI page macros

I’d like to show a couple of icons in my macro; I’ve read the documentation here: https://developer.atlassian.com/platform/forge/custom-ui/#icons, but can’t really follow what I need to do.

The first icon I’d like to show is a ‘sort’ icon; for example this one from FontAwesome: https://fontawesome.com/v4/icon/sort. I can download an svg file, but I don’t know where to put that or how to reference it in my app.