Interact with 24 workbenches.There are 152 collectibles in the game, 19 in each of the 8 main missions (there are none in Mission 9, ‘Loose Ends’). They are divided into five different types: personal letters, classified documents, hidden items, dead-eye targets (stone eagles), and workbenches. They are saved as soon as they’re picked up, so if you happen to die, you do not need to collect them again. Also, if playing in co-op, both players get credit for each collectible.
Personal letters are often found on enemies, making them more difficult to locate if the enemy is on a patrol. Tagging them with binoculars will reveal if they are carrying anything. Some classified documents offer intel on mission objectives, found on desks, tacked to a wall or in safes, whereas hidden items are just trinkets and artifacts. Stone eagles are usually located at the tops of structures or along cliff sides. The most important collectible, however, are the workbenches. There are three in each mission, one for each type of weapon, and finding them unlocks attachments for that type of weapon and allow you to change your loadout mid-mission.
Note: For the workbench collectibles, to earn medal and achievement progression, you must interact with the workbench yourself. Plus, interacting with it unlocks attachments for weapons, which can be beneficial.
For individual mission videos showing the locations of all the collectibles, refer to THIS THREAD, courtesy of 360GameTV.
Note: As DLC missions are released, the requirements for this achievement will not change. If you interact with any workbenches in those DLC missions, unlike the other collectibles, they will not count towards this achievement. The reason why is unknown.