In total there are 60 diaries, 22 sniper nests, 20 cards, 15 weapon parts, and 8 longshots. Diaries and cards will always be where they are marked with no exceptions. Longshot targets never move so it does not matter if the entire rest of the map is alerted, if you miss or hit him several times, etc.
Also required for this achievement are completing all of the optional objectives in each mission. I always completed/discovered these by accident while simply completing the primary objectives and searching out collectibles, so you should not have much difficulty getting those done in the course of chasing collectibles around.
Weapon parts are all found by looting enemies, but fortunately, the same enemy always holds these parts. Some enemies are stationary and some are mobile, but their locations will only mean anything if the area is not alerted to you. Once they start dashing around, hiding, and attacking, all bets are off. Fuck knows where your weapon part went.
Rings indicate that there is more information, so read on below the image if you need help.Mission 1 - Siege of Tobruk
Mission 2 - Gaberoun
Mission 3 - Halfaya Pass
Mission 4 - Fort Rifugio
Mission 5 - Siwa Oasis
Mission 6 - Kasserine Pass
Mission 7 - Pont Du Fahs Airfield
Mission 8 - Ratte Factory
The original source of the plain map images is apparently gryonline.pl. I lifted them from GamePressure.