This can only be completed in a Ranked/Casual playlist NOT in Terrorist Hunt.
There is no real way to guide you through this achievement, Headshots are are easiest to get when aiming at the centre of the chest and letting the recoil move to the head or shooting someone in a **DBNO state.
A headshot is a one hit kill while a body shot can require 3 - 7 bullets so there is some room for error. Operators that can help with slowing opponents for you to line up a clean shot are;
Ella (Grzmot Mine), Echo (Yokai), Blitz (Flash Shield), Ying (Candela),Zofia (KS79 Lifeline) and Blackbeard (Rifle Shield).
Maverick (Breaching Torch) can be used to quietly open sight lines through all breachable and reinforced surfaces for easy headshots on unaware enemies.
Mira (Black Mirror) can use her black mirror on or next to a soft wall, line up the shot, move out from behind taking said shot and hiding again.
Frost (Welcome Mat) The Welcome mat doesn't kill an enemy and holds them in place in a **DBNO providing for easy headshot.
**Down But Not Out