You can unlock this as early as DNA Sequence 4. Occasionally while exploring a city, you may notice a person with an arrow above their head running around like they are being chased. These are Borgia couriers, or messengers. They are dressed in red/burgundy clothes with red capes and elaborate hats and are very skilled at free-running. When you locate one, they will often cry out for help and run away from you. As you chase them, they will try to avoid you by climbing and jumping across rooftops. These couriers do not show up on your mini-map, so if you lose track of one, it’s best to wait until another opportunity comes along; there will be many.
For this achievement, you must subdue a courier, not kill him. I do believe that killing them will give you this achievement, but it will increase your notoriety by 50%. As you are chasing him, when you think you are close enough, press to tackle him. This will prompt him to give you his florins and unlock the achievement.
If you are having difficulty catching up to one, you can use a throwing knife to make him stop. This will not kill him and give you enough time to reach him and take him down.
Couriers are not to be confused with pickpockets, who show up as a icon on your mini-map. They are equally skilled in free-running and are dressed usually in grey or brown with no cape or hat. Subduing a pickpocket will not unlock this achievement.