Instructions
Enter the castle. Go through one of the unmarked doors beside the staircase and follow the Big Boo into the courtyard. Hit the largest ghost and jump into the cage.
Go into the door of the house.
Navigate each of the rooms on the first floor, defeating any Boos you come across with a punch with the "B" button. The first floor of Big Boo's Haunt has five Boos to defeat. In the room with the Boo portrait, jump up into the blue "!" block next to it to get the Invisible Cap and walk through the portrait.
Go back to the main room to find the Big Boo. Run behind it to hit successfully; if a Boo sees you, it is likely to disappear so you can't hit it. Repeat three times to defeat the ghost and open the staircase to the second floor and the first Power Star, which appears at the top of the staircase when you defeat the Big Boo.