Can someone help me with SBD Wolf's Cuphead Debug mod? I did everything what it said here:

https://github.com/DemoJameson/Cuphead.DebugMod

,but it wouldn't work.I'm on version 1.3.4 with DLC.

This program requires BepInEX to be installed.

So I need to install BepInEX and then follow the instructions?

Yes, that is what you need to do.