At the Vs Character Select Screen both players hit & hold UP+Medium Punch

At the character select screen, pressing up or down on the joystick will change the costume color of your current highlighted character.

On the character select screen...

Pick any character. At the match-up screen before the fight begins...

While on the title screen push the following button sequences:

On the character select screen, Hold U or D with the specific buttons as shown below. The button you press will also choose your character, so be sure the fighter you want is highlighted when you do the code. In two player mode, The first person to pick their character gets to choose the stage. The second player gets to pick the music according to the same button combinations used for the stage select.

In VS mode, when you run out of life and your opponent has the opportunity to finish you off, start hitting random buttons like crazy. There is no specific sequence. If you manage to hit the buttons fast enough before your opponent hits you again, you will come back to life with only a sliver of health, but your attack power will be enhanced greatly. If you get hit again, it's over.

Fight your way up to Eyedol, then let him kill you. At the game-over screen, let the timer run down and then go to "Practice", pick your character, then you'll be practicing against Eyedol.