To install this mod, make sure you have the vanilla UNDERTALE version 1.08 available.
Go to https://www.marcrobledo.com/RomPatcher.js/ to apply the patch.
In the ROM file selection, pick the data.win file in your UNDERTALE 1.08 folder and in the Patch file, choose the bps file.
Replace the vanilla 1.08 data.win with the new patched one, and make sure to rename the new one to "data.win", otherwise the game won't work.
That is all, have fun.