On this page of the Far Cry 6 guide you will find a detailed description of The Emerald Skull Treasure Hunt available in the El Este region.
You will start the mission by reading the note that is on the table, on one of the terraces. The game will then lead you into a cave.
In the cave you will have to go straight for the skull, without which you will not complete this quest. You must climb rock ledges and use a rope to climb. There are also some sequences involving swimming as well. Eventually, you will reach a corpse holding a skull in his hands.
Your task is to take the skull and then destroy the boards that block the exit from the cave. You can easily do this with a launcher (but be careful about distance and your health).
You will then proceed to another cave. This one will also be marked on the map for easy reference.
In the cave, you will use the skull that you have acquired. It allows you to open certain passageways and change the position of platforms.
Eventually you will reach a small tunnel, the end of which is blocked by boards. Destroy them with a machete or a gun. When you go through it, you will see a statue hiding a chest in its arms. Opening it will complete the mission.