There will come a point in the main story where you will be asked to kidnap someone and bring them back to them, this is your introduction to bounty hunt missions. This mission will give you all the details but basically you have to reach the target undetected and press
to kidnap him/her. After you have completed this the bounty hunt missions will appear all over the map and you have to do three of these, while bringing the target back alive, for this achievement so it is missable but don't worry as there are about 15 of these missions.
Some tips to make this slightly easier. There is a skill in the stealth tree, 3rd one down 2nd from the left, called Intimidate witch will stop them from trying to escape fro you when you kidnap them. The best way to go about it is eliminate as many enemies around the target before going for the kidnap and if the target runs just chase them down and tackle them,