***If this does not unlock after completing all the secondary objectives like it didn't for me, then you will need to uninstall your game and then re-install it. As soon as I got in the menu after the game was "ready to start" the achievement popped.***https://forum.psnprofiles.com/topic/56983-following-orders-t...
For this Achievement you have to complete all secondary objectives in the main campaign. There is not difficulty requirement so I advise doing this at cadet or to your preference.
You can check your progress from the main menu by going to campaign and chapter select. If it has been completed you will see a green check mark. Some missions have so many that when you are in the mission you can't see them all. The easiest big picture view is from the main menu.
Thanks To Gerbasgamer :"To everyone who still having problems unlocking the Following Orders trophy, no its not glitched and there's no need to re-install the game. I had the same problem. Just head to mission 7 , and go to the bridge leading to the graveyard there's a patrol boat thats roaming there and will appear as a secondary objective once you see it. You'll have to destroy it. Really weird that its not showing unless you discover the boat yourself, I think its the only secondary objective that you must do that in order to get the objective."