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First start a new game and get eight stars if your choice. Now go and beat mean ol' bowser.
Step 1: More stars!
Heres where things get tricky. grab the bunn in the basement but don't take the star. Pick him up and go to the door into the basement.
Now go to the door and drop the bunny into the front of the door. Mario should apear inside the door. get down on his knees and turn towards the bunny. Now put it down again.
You should be inside the room with bowsers door in it , with the bunny! Now do the same thing on bowsers door. Here's the link to a youtube video of it