Instructions
Press "A" on the Wii remote to select "Grand Prix mode" from the main menu of "Mario Kart Wii." The class selection screen will appear.
Press "A" to select 150cc. The Mirror Class is not unlockable in the 50cc or 100cc classes.
Press "A" to select a character, vehicle and drift mode. These options are a matter of personal preference. Any combination of these options can unlock the Mirror Class. The cup selection screen will appear.
Press "A" to select a cup to compete in. It does not matter what order you complete the cups in. Only the Mushroom Cup and Flower Cup will be available at first. The other cups will unlock after you beat the required number of cups.
Win a gold medal in the cup. Each cup has four races. You earn points depending on how well you do in each race. The racer with the most points after four races will win the gold medal. You do not need to place first in all four races to win the gold medal, but it helps.
Repeat steps 4 and Step 5 until you have a gold medal in the Mushroom, Flower, Star, Special, Shell, Banana, Leaf and Lightning cups. The Mirror Class will be unlocked once you have a gold medal in all eight of the cups in 150cc Grand Prix mode.