Silly easy, just win one game of Kung Foot (walk left in the Main Gallery to select King Foot)
Edit: Thanks BadTarantula, the game is a best of 3, so if you get 2 goals, you're done.
You'll need a second controller, either score a goal on your main controller, or do nothing with the second controller for 20-odd seconds and you win.
Bloop 10GS.