Stand in front of any mirror in the mansion except for the large one that reflects the invisible ghosts. Then, switch to your Game Boy Horror's first person mode and press A to warp to the entrance hall of Luigi's Mansion.

Successfully complete the game and save. Ura-Yashiki (Another Mansion) mode will now be available to the main menu. This mode allows you to play the game with various differences.

Successfully complete the game to unlock everything in the gallery.

Use your vacuum to clean all lamps, chandelier, and vases, as most contain either hearts and money.

While playing a game, hold B + X press Start for about two seconds.