Go to the Simpson's house, then go to Bart's room, go to Bart's room, (second room on the left) Go to Bart's desk, open the left drawer, and take out the slingshot, then it will take you to the Sideshow Bob poster .Then you just shoot the poster.
Bonus Game: Donut Toss
Go into Homer's bedroom and click on the desk. Grab the note from the rightmost drawer of the desk. After you get it go to the power plant (Sector 7G) then do what the note says: pull the lever, pull the rope, and press the light, to play the donut toss game.
Go into the Simpson's house look in the bottom
left hand corner there will be some light.
Click on it, then you will be able to enter the
backyard, where you can play a water balloon game.