01.once you found a stage that you like,(I recommend going to the mugen archive) you will find two files a .def and sff file. next locate the local files of the game, and you should be able to find a folder called stages drag the .def And .sff Files into that folder.

2.right click on the .def File click rename and then copy cuz we need to type in the name of our stage somewhere else click on the data folder and you should be able to find a file called select.def open it with Notepad and then keep scrolling down and you want to get to a point where it says:
;Insert your stages below.
make new column under the existing names of the other stages, type in stages/ then the name of your stage and don't forget to put .def at the end. it should look something like this:

stages/McDonalds.def

and of course don't forget to hit save when you close Notepad.

3.next just open the game and you should be able to find it like any other stage except without a preview image lol

and that's it! you successfully put a custom stage into the black heart now have fun!

one thing I want to note is if you are going to put a custom stage, make sure that you do them one at a time, because for whatever reason some stages work and some don't and if you do them all at once it will probably give you an error whenever you boot the game and you won't be able to know what stage is causing it so you if you do encounter that problem just view Integrity of game files, and it will just put everything back to normal so you don't have to delete and reinstall the game.