Once you have downloaded the zip file, don't unzip it. Just leave it as is
In the Arduino IDE, navigate to Sketch > Include Library > Add .Zip Library
Once prompted, add the zip file.
Return to the Sketch > Include Library menu. menu. You should now see the library at the bottom of the drop-down menu. It is ready to be used in your sketch. The zip file will have been expanded in the libraries folder in your Arduino sketches directory.