Is there a guide for adding third-party CSS to 5.4?

I'm trying to install flag-icon-css to my project. I ran npm install flag-icon-css and it created a folder under /node_modules/flag-icon-css. I'm flummoxed what to do at this point though.

I tried adding mix.copy('node_modules/flag-icon-css', 'public/css'); to my webpack.mix.js config but all that did was flatten out the directory structure under node_modules/flag-icon-css and dump every file in every directory /public/css.

Any pointers?