If you have Kinect hooked up you just say "Xbox record that" and the last 30 seconds become a clip. If you don't have Kinect you press the guide button and then X and then it should make you a 30 second clip.
http://xboxclips.com/ is where I look at my clips. you just search for your GT in the search bar. On the bottom right it says Download. click it and the clip is now yours.