I'll add Rainbowdash in later, but I know some people might want to have one with Derpy separately.
1. Download with your favorite youtube downloading program.
2. Insert clip into favorite video editor.
3. Find the chroma key option in video editor (not everyone program has this).
4. Tell the program to remove all of the light green.
5. Place random video/picture background behind.

I would appreciate it if you gave me credit for the rotobrushing.
Program I use for chroma key: Cyberlink Power Director