Before you can do this you must play till the end of the story where you get the ability to command Uruks. You need to brand a captain and command him to infiltrate a Warchief. This does not always unlock a recruitment mission though. Its random what type of mission you get, so just keep commanding captains until a recruitment mission shows up on your map.
In this mission you need to defend your captain and kill other Uruks. After successfully beating the mission you need to kill your captain and all his recruits.