Instructions
Start a new game in "Mario Kart Wii" and select the 150cc engine class for whatever vehicle you choose to race with.
Obtain a 1 star rating for each of the 8 Mirror Cup races. A one star ranking is based on your driving technique, which factors in your wins, track times, trick points, and errors made, among other things.
Save your game after obtaining 1 star rankings for all Mirror Cup races. Go back to the character selection page and Rosalina will be unlocked and playable.