Just go into documents/my games/afterbirth + mods and paste the modfolder in there. (make sure that you have the "resource" etc. files in the sub folder directly)
Doesn't works, im playing in Afterbirth + and i tried the original method of taking the file to the "resource" file but it doesn't works, do i need to take it to the "tools" file instead?
Sad but imo justified.
Modding shouldn't make getting achievements easy.
It does suck for music or graphic mods though.
If not, I'm sorry
But it's as simple as replacing one file with another, which you could do yourself (boss fight intro jingle_01).
You can find it on my profile hopefully.
Then you should be able to locate it in the main menu.