The first thing that you need to do is to insert Mario Kart Wii into the Wii console and start the game. Go to the single player menu and choose the “Time Trials” mode from the list.
Once on the character select screen, select any character you wish to race as. Afterwards, choose the track from the list of courses that you wish to race on.
After you are done choosing your character and the track, select the ghost data which has a name beginning with “Nin” or “Nintendo”.
Once the time trial starts, you will see the Nintendo staff ghost racing alongside you. It will be quite distinguishable because of its ghostly appearance. You will not be able to interact with it. Your aim would be to not only beat the Nintendo staff Ghost, but do so by a margin of at least seven seconds. By managing to successfully do that, you will unlock the expert staff ghost.
Race against the expert staff ghost and beat him as well by a margin of at least seven seconds to unlock a staff ghost with greater speed.
Continue racing against and beating the unlocked staff ghosts on seven more courses. Remember to defeat each ghost by a minimum margin of seven seconds.
Once you have successfully unlocked all eight expert staff ghosts, Baby Luigi will become available for selection on the character selection screen.