The main doc set is probably where to start. It’s not a small topic.
https://sass-lang.com/install
https://sass-lang.com/documentation/file.SCSS_FOR_SASS_USERS.html
You can compile it in various ways, such as using gulp or some GUI app like Codekit on Mac.
But frankly, it’s probably easiest to actually figure out how to use hugo pipes from the first, since it does the work for you. That means, figuring out how to adapt the theme or working w/ the theme author to upgrade it to use Hugo Pipes.