Using the standard controls when you launch the game. To mark an enemy, you must aim at them with
and
you will see a small red diamond appear.. If you don't see the diamond it will NOT mark the enemy. If you hit
you can mark the enemy so they stay visible for a few seconds.
Best mode to be playing is any Big Team Battle. More players running around means more chances to get it unlocked.
You will know if you missed with a normal white icon with the areas name on it. These do not count, you must be marking an enemy player.
New note... I have noticed that when using the disruptor pistol, it actually marks enemies, now I cannot confirm myself if it counts, but the lightening shock can touch other Spartan players with the mark too. Maybe you can use this to unlock it. If anybody can confirm it would help loads I think.