At the beginning of the level, move forward, hugging the right wall and curving slowly left in the cave up ahead. After a short amount of time, you will see a tunnel opening to the right. Move through this, but still hug the right wall. Part way along an opening (cave) on the right will open up.
Move inside, turning your flashlight on. Soon you will reach a dead end, and the skull is on the ground, among the rocks.