Instructions
Select the Grand Prix mode on your Mario Kart Wii main menu.
Select the Cup that you wish to race for. Depending on the cup that you choose, the series of tracks you race on will be different.
Race on the first track, and get either first, second, third or fourth place. You will be allotted a certain number of points depending on your finishing rank (for example, if you get first place, you will receive 15 points. If you get second, you will receive fewer, and so on).
Repeat step 3 with the next three tracks. If you have more points than the other racers at the end of the four races, you will receive the cup. Once you receive the cup, you will unlock a new cup, along with a new series of tracks.