You can download the installer to do the work for you. To do so, just download it from here, unpack the archive and launch the .exe file. Antivirus might think that it is a virus, just allow it on the device if it gets triggered. I also recommend to choose the new installer instead of the old one, because it is more easier to use.
First of all, you need to install BepInEx. To do so, go to BepInEx 5.4.21 release and download BepInEx_x64 (or BepInEx_x86 if you are running a 32 bit system).
Move the contents of the folder in the game folder (where BALDI.exe is located) and launch the game once to generate all required folders.
Head to the releases page and download mod.dll.
Move mod.dll to %GameFolder%\BepInEx\plugins
Enjoy!
If you have a suggestion, open an issue!