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. The dark skin will not work if the regular skin is not also installed:
\Documents\Sports Interactive\Football Manager 2021\skins\
Run Football Manager 2020 and go to Preferences > Interface.
You should see "FMSCOUT-FM21-Instant-Result” or "FMSCOUT-FM21-Instant-Result-Dark" as an option in the skin drop down of the General options section.
Hit the Confirm button.