Instructions
Insert Mario Kart Wii into your Wii console and power on the game.
Select "Time Trial" mode on the main menu.
Select a player and vehicle to begin the time trial.
Complete a time trial competition, which requires three laps around the track. If you set the best time on the track, you'll unlock the "ghost" for that track. The ghost is a partially see-through character that you race against; you can pass through the character on the track, hence the name "ghost."
Start a time trial again on the same track. You'll now compete against the ghost during your time trial.
Defeat the ghost that you just unlocked. If you beat him, you'll unlock a second ghost called an "expert ghost." Each track contains one expert ghost. The expert ghost is a more difficult version of the normal ghost, and it represents the best time trial time for the game's developers.
Repeat Steps 2 through 6 on seven more tracks. When you have finished, you'll have unlocked eight expert ghosts.
Go to the character select screen. Once you've unlocked eight expert ghosts, you are now able to select Baby Luigi as a racer.