When you begin the game, this can be a little intimidating as you learn to dispatch individual foes but as you progress through the game, you'll likely manage this just through playing. If you are having trouble then use the "Zack" mask (frog) which increases the amount of time between kills to continue your combo streak. This can be gotten by getting a high score on level 13 (the exact score is unknown but at least a B+).
This can be done pretty easily on chapter 5. Right at the beginning, you'll see a man in the second room with a crowbar. Grab the crowbar, kill him and then kill the man in the adjoining room. From here, go back to where you picked up the crowbar, head through the south door into the hallway. Go down to the end of the hallway and turn into the bathroom on the left side where there are two men. Kill them quickly to get this.