U know the courtyard Go there (u need 20 stars or so though). Ground pound (kill) the big Boo and enter the bird cage. Go into the mansion, through the main entrance, and get onto the balcony thing above the first hallway. Go into the room furthest to the right and get onto the stage. Using Mario, get up, onto the platform above u, and go through the door. Get into the Luigi painting. Go through the door to the right, then left, and so on until u get to a room with a massive collapsing bridge. Go across it, and then use the green lifts to get up into a hole, go in. Look in the mirror (zoom in) and defeat King Boo, to unlock Luigi.

P.S. Ground pound him and it has more effect.