spoiler alerts though it is a "captain obvious" thing......
After rescuing Luigi (go to 3rd floor of Boo's Mansion) bring Luigi to the 2nd floor of Toadstool Castle and enter the room containing the Snow Man's Land painting portal. Use the power flowers to go to thorugh the mirror and enter te wario painting. Also,btw try looking through the door behind the mirror for another nifty new secret for SM64DS (just a hidden star,but somewhat nifty none the less)
Happy gaming
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Working out for an hour and a half makes you get bigger muscles. Writing for an hour and a half makes you a better writer
~Stephen King