Collect the first dog tag.

At the beginning of Chapter 7 (you can check what chapter you're playing by pressing Start > Save and use a save-slot)
When you are back at the camp you were before, but the sky is now green-ish
You need to find a small stone monument with a lit candle, it's very straightforward because wherever you decide to go, you'll always pass that candle. Once you found it, just press RT to interact with it and your character will find a dog tag.



Achievement is worth 15 Gamerscore.