Instructions
Mii Outfit A
Select "Grand Prix" from the main menu.
Select "100cc."
Select "100cc Special Cup." To access the Special Cup, you must have first won the Mushroom, Flower and Star Cups.
Place first in the "100cc Special Cup" to unlock Mii Outfit A.
Mii Outfit B
Select "Time Trial" from the main menu.
Select one of the 32 tracks. The time to unlock the expert staff ghost will be shown.
Race on the track and beat the lap time shown earlier to unlock the expert staff ghost. Unlock the expert staff ghost on all 32 tracks to unlock Mii Outfit B.