This achievement is done on the mission Airport. You have to upload an antivirus from two different computers to your phone and then upload the antivirus from your phone to the main servers in the airport. You have to move throughout the airport without being seen by any enemies. Stay crouched the whole time so enemies can’t hear you. Always use your silenced pistol to kill enemies from far away and get a head shot on them and when enemies are close take them down by clicking RS. Disable all the cameras throughout the airport by finding a yellowish box on the wall around them and pressing X when you are next to the box. Once you upload the antivirus to the third computer the achievement should unlock.
If an enemy is alerted, just restart from the last checkpoint and you will still unlock the achievement.