itch.io

I think you need to enter the the files and extract that file(There will be an extract option after you enter the files) and after extracting the file a new file will appear and you have to enter that new file where there will be the file for the game(you'll recognize the game file right away because it's icon is different from the rest)

The installation guide can be found on the DLC's Itch page! Also, the DLC won't work on its own; you need to install it with the base game.