After you return the fortune teller's crystal ball, she helps you get into Shecola castle, where you get instructions for the west cave.

Explore the Cave

From the entrance, go north to the next room.

Jump 10 times on the tile (shouting "Abracadabra" is optional) then go north to the next room.

Take the two double hearts, then jump west and kill the Octots. Then go south through the door that opens.

Kill or ignore the Octots, and go west to the next room.

Go west, but be careful of the Mud-o-Fish that jump out of the water and run toward you. Keep going west to the next room.

Mud-o-Fish will jump onto the island in this room. Kill the Octots, then go north and south to make the Mud-o-Fish come out so you can kill them. Then go north to the next room.

Keep going north to reach the fifth part of the overworld.