iPad powered up. Apple tv powered up. Both devices on same wireless network. Double tap iPad home button. Task bar appears at bottom of screen, swipe your finger over the task bar area to your right. Directly to the left of the volume slider is the AirPlay mirroring button. Press it, enable mirroring.
Glad you asked this question, I just got the new Ipad & the Apple TV (which I haven't hooked up yet) & was wondering the exact same thing, in fact that's the only reason for purchasing the Apple TV too.