For this Achievement you have to collect all 17 Battle Journals
, all 16 Voice Mail Recordings
and all 34 Notes
. The 100 Zombie Statues and 30 Flags are not required for the Achievement. You can check in the Inventory Tab (push X here) which Collectible you have collected and which not. Battle Journals and Notes are located on Slums and Old Town, Voice Mail Recordings only in Old Town.
The best Time to collect the 67 Collectibles is after the Story. Choose Continue in the Mainmenu and you go back to Harran in Free Roam Mode and can all do what you want. Another good Hint is, reach Rank 12 in Survivor Tree for the Grappling Hook. This Tool makes the climbing so much easier in the Game.
A Word to the Sidequest, Sidequests unlocks Locations and on this Locations are many Collectibes. For the Battle Journals you have to play 0 Sidequests, for the Voice Mail Recordings you have to play 2 Sidequests and for the Notes are tons of Sidequest to play, so the best Way is, beat the Game, play all Sidequest and then begin to collect the (missing) Collectibles.
Note from Prattalmighty in the Comments:
You may want to also add in a part for the people who were stuck like me with 67/67 collectibles found, but less than 100% showing in the tracker. They can start a "New Game" save file and start picking up notes and battle journals, they'll see their completion percentage rise and the achievement will unlock after a few, work for me today.Battle Journals
Voice Mail Recordings
Required Sidequests and Quest Giver: (Video sorted)