Instructions
Start "Mario Kart Wii" and open the single player menu. Chose "Time Trials" from the list of gameplay modes.
Select any character, then any track from the list of available courses. Choose to race the Nintendo staff ghost by selecting the ghost data whose name begins with "Nin."
Beat the time set by the staff ghost by at least seven seconds to unlock the Expert Staff Ghost. The ghosts will be racing alongside you, but they are transparent and cannot interact with you. Use the three boosts you are given at the start to boost through straightaways or cut corners by boosting over large patches of grass and dirt.
Continue beating the staff ghosts on seven more courses. After unlocking eight Expert Staff Ghosts, Baby Luigi becomes available.