Follow the path down the hill from the base camp. As you approach a small stream, Lara notices a corpse hanging from a tree, and it's holding something you can use. Approach the bunker on the other side of the stream, move close to the quiver of arrows and press Interact to add them to your inventory. There are more arrows leaning against the tree from which the man is hanging.
Climb on top of the bunker.
Then jump across the small gap and step out onto the branch lodged in the fork of the tree near the dangling man.
After the brief cut scene, in which Lara loses and regains her balance, lean to the right to reach out and grab the MAKESHIFT LONGBOW.