For this, host or join a "Capture the Flag" ranked match. To save flags, a player on the opposing team must take one of your team's flags. Once that player is holder your flag, for him to drop the flag you or someone on your team must kill him. Once the player is dead, the flag is dropped. Now run over the flag and automatically you save it. Now for the achievement you must save a total of 50 flags, not in one game, but if you can do that I am impressed. Good luck!