Instructions
Start the game and press the "A" button on your Wii remote to enter the Main menu. Point at the Time Trials option and press the "A" button.
Select the type of character you want to race as by pressing the "A" button. Select a type of kart or bike to use in the race from the Vehicle screen.
Move the Wii remote to one of the Cups and then a list of courses will appear. Point your Wii remote at one of the courses and press the "A" button.
Click on the "Ghosts" option and then click on the "Race This Ghost" option. Complete the race by beating the ghost's best time. This will unlock the expert staff ghost for that particular course.
Go back to the Course menu and race against another ghost. Continue racing against ghosts for Time Trails courses until you've unlocked eight expert staff ghosts. A pop-up will appear on your screen saying that Baby Luigi has been unlocked.