At the beginning of the level, if you go into view mode by pressing up on the right stick, you can look up and see a Big Boo near the roof. That is where you will be heading for this star, so head inside the main mansion. Go up the stairs and enter the last door on the right side, placing you in the room where you got the final Red Coin during [Seek the 8 Red Coins]. Stay on the right side of this room for now and locate the raised floor. If you Double Jump near the top edge, you should catch a glimpse of a ledge above you.
To get to it, Wall Jump at an angle off of the right wall and you should be able to grab the ledge. Should you need some extra height, try a Side Somersault into a Wall Jump. Go through the door in front of you to find yourself in a new room, with a Blue Coin Block. Ignore everything here and locate the door on the right side to exit to that Big Boo you saw at the beginning. Defeat the Big Boo like normal, but you will notice the star goes on the roof above you, which is a bit out of reach.
To get to this star, you will need to do some Long Jumping, but the camera will not be any help. Look to your right to see another part of the roof, which is where you will be jumping to here. As soon as you Long Jump, hold forward and don’t let go until you’re on the top part, where it is stable and Mario won’t slide. From here, you want to go to the highest point of the roof, where it is flat like this. The Mario camera (press the R button) helps out a lot in keep on the flat portion here. Move over to the spot where the star is located, then slide down and grab it.