Once you obtain 15 stars (it is possible with 14 but much harder), go to the courtyard and the cage that leads to Big Boo's Haunt. Remember that choosing one of stars 2-7 is recommended for this. When you arrive, enter the main mansion, go upstairs, and enter the door on the far right. You should arrive near a red ? box. Stand on top of it, and backflip or wall-kick to the 3rd floor.
Enter the door, and go into the Luigi painting after that.
What's this? You seem to have arrived at a merry-go-round maze. Here, if you go through the wrong door, you will end up at the very start, and if you fall down a bottomless pit, the same thing will happen. You can't die by either of these ways.
Instead of going back and forth, take the door to the right when you enter. This should lead you to another area. Go to the left door this time. Again, another room. Go to the right, then left. Then run like the wind until you reach 3 platforms. Get on them in order, and go in the hole.
You get into a room with a large mirror. Go up to it, and King Boo will appear! Either punch him from the behind or ground pound him 3 times. He will give you a Luigi key, which opens up Luigi's door.
Get all 8 red coins. You need to have unlocked all characters and the ? Blocks to do this.
- Right when you start, there is a red coin on the far side of the room.
- Get the Mario cap next to the left door if you aren't currently Mario. Go out the right door, and you can see two coins. One is on the corner of the middle platform, and one is on the right bridge.
- Go to the top door. The right platform has a coin on the corner.
- Go to the middle platform - be careful, the center of the middle platform tilts all over the place - and grab the Power Flower within the ? Block, which lets you float. Float to the top-right corner of the room, where another coin lies.
- Head through the right door. There are four checkerboard platforms here, and the first and third ones lead directly into coins, so just ride all the platforms to the end and back. Get the Wario cap while you're at it, or get hit by an enemy if you came in as Wario.
- Go through the top door. Straight to your right is a Black Brick Block containing the final red coin.
The star is on the same platform as the last red coin, but on the other side.
This star is hidden behind a painting. You need to have Luigi to get this.
Go into the third room, and grab the Power Flower. Now you have to run fast. One mistake and you have to do this all over again.
Once you have invisibility power, run all the way to the room with the very long bridge. Instead of jumping on the platforms however, keep running and jump straight through the painting, where the star awaits. You can get out just by jumping out.