Instructions
Click on "Start Rockin'!" at the main menu. Click on any of the songs to start. It might help to choose a longer song, if you're having trouble hitting 1,000 mistakes in one song (the track time is listed below the song name).
Press the directional arrow keys or the number keys rapidly. Individual directional keys and some numbers will flash on the screen for you to play along with the song, but your goal here is to not hit the correct buttons. Keep hitting different directional keys until you have over 1,000 mistakes. When you hit the 1,000 mark, the number of your mistakes will change to "Lots."
Click on "Main Menu" after the song has ended, then select "Change Guitar." The Butterfingers guitar is the fifth guitar from the right.