Big Boo's Balcony is the fifth stage in Big Boo's Haunt, which is the fifth level in Super Mario 64. The level is accessed through the castle's courtyard, rather than by jumping through a painting like most other levels.
Enter the house
Begin by walking into the haunted house.
Go up the steps until reaching the second floor balcony.
Take a right, and then walk to the very last door. Enter through that door.
Access the platform
Walk past the bookshelf until a platform is visible near the bottom of the screen. Jump onto the platform.
Perform a special jump to reach the third floor
The third floor is hidden, which is why it can be difficult to access. Starting from the platform, perform either a wall jump or a triple jump to get to the third floor. A wall jump is performed by pressing the "A" button as Mario slides down a wall. A triple jump is performed by running and then jumping three times consecutively. Each jump goes progressively higher, allowing Mario to reach high areas.
Defeat Big Boo
Go through two doors, and then fight Big Boo. Face away from him, and then perform a double jump when he is close. When he floats under Mario, land on him with a ground pound. Do this three times to achieve the star.