Once you have downloaded the folder (usually titled Source_Sans_3 or Source_Sans_Pro ), follow these steps:

It features a large x-height, making it easy to read on small retina displays.

The easiest way to get the web and desktop versions. You can download the entire family as a single .zip file.

This is the official home of the font. It is the best place to find the "variable" version of the font. How to Install on macOS