Make sure you turn on movie capture option in options and then while playing the game press 'F2' which will capture the last 40 seconds of game footage. But i have noticed it sometimes and in most cases captures more than 40 seconds, around a minute or so.
Then when you've captured some footage goto your mobile phone and goto the video editor and make yourself a movie. But be careful don't go mad with the footage cos it can take a while to put it together. Try about 3 or 4 clips at first and see how you get on.
