What does select filter do

1: Download any song in-game. Remember the song ID. It should be displayed in the "Saved" folder.

2: Get your .mp3 file.

3: If you are playing on a Windows computer, go to "C:\Users[username]\AppData\Local\GeometryDash" and replace the music file with the song ID you remember with your .mp3 file.

4: If you are playing on a Mac computer, go to "~/Library/Caches/" and replace the music file with the song ID you remember with your .mp3 file.

5. Use that song in-game that you changed.

6. Have fun!

(Note: This is not universal and it will only affect your song. No one else will hear the same .mp3 file as you do. Other than what I've described, there is no other way to get a custom-custom song)

Go to www.newgrounds.com, find a song, copy the last part of the URL at the top. That is the ID for the song. Next, go into the level editor and tap on the cog in the top right. Then, tap the custom button, and hit "New". Finally, type in the ID you copied into the text area that says "Enter song ID".