Instructions
Insert the game and wait for the title screen to load. Use the d-pad to highlight "New Campaign" and press the "X" button to accept.
Select the number of players and select the difficulty level. On the "Easy" level, enemies are easier to kill with melee attacks. On harder levels it will be trickier to get close enough to perform a melee attack without getting killed.
Press "X" to zoom in on the characters. Press "left" or "right" on the d-pad to cycle through the choices and "X" to show the character bios. Private Stephan Frost, the character on the far right, specializes in hand-to-hand combat so is the ideal choice for a melee attacks.
Press "X" to select the character of your choice then wait for the level to load. Press "Up" on the d-pad to deselect your currently equipped weapon. You will notice the weapon icon on the bottom left of the screen is greyed out and the weapon will no longer be in your character's hands.
Walk up to an enemy using the left analog stick. When you are close to him, press the "L3" button to make your character crouch. There is an enemy to the left of your location right next to the cart that you can practice your melee attacks on.
Press the "R1" button all the way down to perform the melee attacks. Keep pressing "R1" until the enemy is defeated. Make sure you are close enough before performing the melee attacks, otherwise the enemy will hear you and turn around, opening fire.