I tried both guides shown here and neither worked for me. (not saying that they don't work, they just didn't work when i tried them)
I got this today by sheer luck but I tried it a few more times and it seems pretty simple to do.
difficulty: amature (probably be just as easy on a higher difficulty)
Get towards the edge of the oppositions penelty box and turn to face your goal, there is usually a defender stalking you from behind kick a shot at him about 1/2 to 3/4 power when he is a few feet away from you and it should slip through his legs.
granted this won't work all the time everytime but I tested it out over several games and it seemed to work on average once a game.
I know the other two solutions have worked for a few people but if you are having trouble with them like I did I am offering an alternative.
If negative voting, please leave a comment to explain why.