Bonus Stages
Beat any 2 stages in the normal game to unlock 2-player Lammies.
Beat 2-player Lammies to unlock 2-player vs. Lammies.
Beat the 1-player game to play as Parappa.
Beat 1-player Parappa to unlock 2-player Lammy Parappa.
Beat 2-player Lammy Parappa to unlock 2-player vs. Lammy Parappa

Get Bonus Parappa Levels
Finish the game, return to the level select screen and press Right until the bonus levels appear.

Custom Lyrics & Notes
As soon as Lammy appears on the title screen when booting the game, immediately reset the Playstation system. When the game restarts you should have a new menu next to Lammy's guitar on the title screen.

Hints
Practice, practice, practice. Your controller is your instrument. You have to have the buttons memorized and be able to read the symbols displayed on the screen as though they were a musical score.

If you feel as though you're tanking halfway through a level, don't give up. The easier riffs will usually repeat later on, and you may be able to recover enough points to clear the level.

For very rapid runs of notes, rather than trying to press the gamepad buttons individually, slide your thumb across them. In this way, you can go from note to note without having to lift your thumb off the controller.

When you improvise, be sure that the last note you hit is right on the beat, or you may not get any points for that turn. Try to hit as many buttons as you can as quickly as you can to get the most points. Most of the time you don't even need to pay attention to the symbols on the screen as long as you keep up a decent rhythm and pace.

Use the final level to really cut loose. It's the only one that really gives you enough time between measures to jam properly, and the riffs it gives you to play along with are so minimal that you'll need to supplement them to get a decent score.