Move the downloaded .fmf file to:
\Documents\Sports Interactive\Football Manager 2018\editor data\
Create folder "editor data" if it doesn't exist already.
Run Football Manager 2018 and start new save with this database enabled.
If you want to play in the English lower leagues, it's highly recommended to disable "Do not Add Key Staff" and enable "Add Players to Playable Teams" in advanced options. And also load all players from England using the Advanced... Database Size options.