It is very simple if you follow the instructions. All you need to do is simultaneously shock 5 enemies with electricity. This can easily be done with electric traps. You can find one very early in the game, on a street north of the safe zone where the game starts (Slums). Throw some firecrackers to lure the infected to the trap. When 5 or more zombies are in the trap you must activate the power switch to kill them.