Step 1: Collect custom music in BRSTRM format in a folder on your computer
Step 2: Have
http://www.lakora.us/brawl/songmanager/ installed on your computer
Step 3: Open Song Manager and navigate it to the folder of the PM build you want to add songs to( just click the 'projectm' folder and hit OK)
Step 4: Also open your folder mentioned in Step 1, so that you can see both things.
Step 5: Click the song in Song Manager that you want to replace. Drag and drop from the music folder into Song Manager.
Step 6: Repeat Step 5 for all music you want to replace. Add song titles in if you like.
Step 7: Save your changes using the menu to update the .pac files
Step 8: Put your custom build on your SD card/Dolphin and enjoy your new music.
DO NOT just open the SD card build in Song Manager, or you are 90% guaranteed corruption. Make a backup of your SD card on your computer, navigate to that folder in Song Manager, make your changes, then erase all files on your SD card, and replace it with the modified backup.