Dear Diary... Achievement in Halo: The Master Chief Collection

In total, there are 10 terminals spread across each of the 10 missions within the Campaign that must be activated for this achievement. These terminal stations are well-hidden and usually off the main path of your objective, so it will take a keen eye and some exploration in order to track them down. You’ll know when you’ve found a terminal by the spotting a bright yellow hologram with a flashing panel. Simply approach the panel and hold the action button to activate it.

Take note that you do not necessarily need to watch the entire terminal because progress towards this achievement registers as soon as it’s activated. It is highly recommended to spend the two to three minutes in observing each terminal though. Once activated, they will provide crucial backstory to the Halo Universe. Also, take note that terminals will not appear while playing in Classic Mode. Once all 10 terminals have been activated, this achievement will unlock.