Instructions
Begin with the Haunted House in front of you after you collect Star 4.
Enter the Haunted House.
Head up the staircase to the second floor.
Turn right. Walk until you reach the last door on your right, and enter the room.
Jump on the secret platform located just past the bookshelf to your left. You can see it by pressing "C+Up." You will need to do a wall kick to clear the jump.
Enter one of the two doors ahead. Both are entry points to Big Boo's balcony.