This is very easy to do on the very first mission of the game. After you complete the tutorial you will be on a beach were you can practice shooting. After that you will use a grenade to get past a iron-link fence door. After another cut scene you will start the actual level with a guy right in front of you facing the opposite direction (and he never moves unless provoked). Just take aim and fire. You are going to want to aim at the center of his rear end horizontally, and fairly low vertically speaking. You will know you got it right when the soldier grabs his rear end and falls to the ground. If you miss you can press start, left trigger, and select restart mission. When you restart the mission it starts you off right behind the same exact guy so you can try and try again until you get it right.