Then download the latest release (the zip archive not the source code).
In this archive, you will find the folder named Monika_AfterStoryversion-Mod. Extract it somewhere on your computer.
Open the base game folder. In this folder, you will find the folder named "game".
Copy all the files found inside the Monika_AfterStoryversion-Mod folder into the "game" folder.
That should be it. From there the game could be launched from DDLC.exe.
how do I open the game file?