Download the .zip archive and use a tool such as 7-zip for Windows or The Unarchiver for MacOSX to extract its contents.
Move all extracted folders to your skins folder:
\Documents\Sports Interactive\Football Manager 2018\skins\
Run Football Manager 2018 and go to Preferences > Interface.
You should see "Wannachup-Instant Result” as an option in the skin drop down of the General options section.
Hit the Confirm button.