Very easy to do, first you must incapacitate any enemy. Can be down fairly easy with a pistol like the welrod (silenced pistol). Shoot the enemy below the waist until they drop to the ground.
You’ll get an indication telling you you’ve incapacitated them. Once they are laying in the ground. Take your sniper rifle out and aim for the testicle area. Can’t miss it. I would stand back 2 meters or so and go for that nut cracker.