Umissable achievement, you get this for completing the first part of the game, after the tutorial and brief prologue.
Major steps listed below (but take your time to explore):
*** Spoiler - click to reveal ***
1. As you enter the beach area, you'll see a metal box raise up on a bunch of lines. Ignore it for now. Turn right and head through the ship to the area by the campfire.
2. Go past the campfire, past the tent, and then head into the cave (picking up the flashlight and following additional tutorial instructions as you go) and up to the main level.
3. Read the note and listen to the cassette tape. As it suggests, pick up an object and throw it at the metal box above your head to knock it down. Pick up the flare gun and flares from the box.
4. Head out to the balcony. You need to shoot three different bells with the flare gun. The first (and hardest) is on top of a pole near the shipwreck, left of the lighthouse in the background. The second is lower to the ground, on a buoy, more-or-less directly between you and the lighthouse. The third is behind you, just to the right of the entrance of the cave. Note that you can also get the missable
here if you hit all three with a single clip.
5. After shooting all three bells, head out of the cave, back past the fire, and through the ship you first entered. Remember that box that raised up when you approached? It is now lowered and you'll find a door handle and another cassette inside.
6. Go back through the ship then go through the entrance to the right to a metal fence and door. Use the door handle on the door, open it, and watch the cutscene and the achievement pop for completing the first part of the game.
Important note: There are a number of missable achievements (including the one listed above) that can only be completed in the beach area before completing this achievement, so you'll need to do them before finishing this or restart the level and do them later.