GordonBM &
Passerby#4554
You don't need the "UnRen.bat" tool or any ".rpa" archive decrypting tool. You don't need to mess with the original game files at all.
You also don't need the ".rpy" files for the mod to work.
All you need to do is create a sub-folder called "script" inside the folder "game" located inside the base-game directory and simply place the ".rpyc" files(myroom.rpyc & flat.rpyc) inside the "script" folder you created and RenPy will automatically override the original files inside the archive with the modded ones.
So, I'd advise you to update your install instructions.