At the character selection screen, hold L or Z, then press A, B, A, B, A, B, C-Left, C-Right, C-Up, C-Right, C-Left, C-Right, release L/Z. Now, pick and character and once the fight begins, press Start and quit the game. If the code was entered correctly, you'll be taken to the debug screen. Use the analog stick to pick a character, C-Up/C-Down to choose a stage, and Start to begin the fight.