1. Download DeltaPatcher, if you haven't already: https://www.romhacking.net/utilities/704/
1.5. (OPTIONAL) Make a copy of your Undertale game files somewhere else in your PC so that you aren't stuck with this mod as your base Undertale. (C:/Undertale, for example)
(Do the rest of this guide in the copy folder, if you made one.)
2. Patch Undertale's data.win with the one in this .zip through DeltaPatcher.
3. Copy the "mus_storyofut.ogg" audio file into the game directory. You need this to hear the music.
4. Mentally prepare yourself for the heavy content you're about to experience.
5. Launch the game.