Couple of conditions must be met for this particular way of getting the achievement to pop on Steam. The achievement itself, the campsite where you get the cassette for this achievement, and the cassette itself are buggy. Sometimes the cassette never shows up. Sometimes it does and picking it up does nothing. Sometimes there is only one bottle on the ground. Sometimes there are a lot of bottles on the ground.

This guide is what I did after trial and error to get it to work. It worked the first time I did this. When you pick up and store the cassette in your inventory, the achievement will pop in Steam.

- You must have beat the game.
- You must have access to Thorofare Free Roam from the Special Features Menu.
- You must have an empty Save Game slot.
- You must have learned what is necessary for fulfillment of the achievement.

Task #1 - Complete the game.

Task #2 - Close the game.

Task #3 - Restart your computer.

Task #4 - Run Firewatch from Steam normally.

Task #5 - At the first menu screen after game loads, select Special Features.

Task #6 - Start a New Game in an empty Save Slot with Thorofare Free Roam.

Task #7 - Upon waking in the Watch Tower, grab ONLY the axe, nothing else.

Task #8 - Exit the watchtower and walk directly to the outhouse by Hawk's Nest.

Task #9 - Secure the 'note' inside the outhouse in your inventory by picking it up.

Task #10 - Go to the hidden campsite behind the bushes and blocked trail, ignore the dead Elk.

Task #11 - ONLY pickup the cassette, which is difficult as there will be a lot of items in the area.

Task #12 - Secure the cassette in your inventory. This step gets the achievement to pop.

I have no idea why this works, but it did for me, hopefully it does for you.

Good luck!