You can perform a stealth kill if you stay unnoticed.

To perform a stealth kill, quietly sneak up on the enemy and press main attack button.

Main attack button will look like this    when a stealth kill is available.

To sneak up on the enemy use  stealth/crouch movement mode.

Look for objects which can cover your approach (like bushes or high grass).

Remember that enemies not only see, but they can also hear you.

Stealth kill cannot be used against some strong and large enemies.