To "install":

1. Navigate to %USERPROFILE%\Documents\My Games\NieR Replicant ver.1.22474487139\Steam\(user-id)\
2. (Optional) Create a Folder within the Save Game location called Backup
3. (Optional) Copy your existing GAMEDATA file into the Backup folder you just created
4. Drop the GAMEDATA file from this Mod into %USERPROFILE%\Documents\My Games\NieR Replicant ver.1.22474487139\Steam\(user-id)\ and Overwrite when prompted
5. Load up the save and play as normal (All three slots have the same progress)