It's super easy, just download, unzip, and copy the "Plumfruit DEFINITIVE EDITION (By Arnie)" folder in your mods folder! That's it! (make sure to remove older versions in order to update properly!)
The Install Directory is usually locatd under:
Make sure you have enabled Mods in your game’s settings! Go to settings/other and check the little boxes “enable custom content” and “enable script mods”