Hey everyone,
For the people who like a visual but don't want to scroll through video's, I have found a very useful website. I did not contribute to this image-guide so all credits go to Nemdan (the writers username on the website listed below. The guide says you don't have to spend $500 on the maps available at shops, but I'm recommending buying them anyway for a more accurate location of the collectible.
http://nemdam.hubpages.com/hub/Far-Cry-3-Letters-of-the-Lost...Good luck!