After you defeat Maxie the Ghost and drain the pond in the Ghost Village, you find the Fortune Teller's crystal ball.

Take the Crystal Ball to the Fortune Teller

Go to the drained lake and take the crystal ball. Then go east to exit the Ghost Village. Keep going east to reach the mountains. Walk into the gap in the mountains and go east, then south into the hidden cave in the mountains. You can ignore the stairs if you got the Big Heart earlier. If not, go down the stairs and get it, then go back up the stairs. Then go south to exit the mountain cave.

Go east and enter Shecola Castle. Go east along the grassy path that goes around the castle, and make your way to the stairs around the back of the castle. Go down them. Talk to the Fortune Teller to give her the crystal ball.

After the cutscene, go north and talk to the guard to enter the castle. Enter the room in the northwest part of the castle and go up the stairs. There will be a cutscene. During the cutscene, you will get the Shooting Star. Your weapon will automatically be upgraded to the Shooting Star if you have at least 6 hearts full. The Shooting Star shoots a spiked ball away from you in the direction you're facing.

Go south to exit the room, then go to the northeast room of the castle. Talk to the woman at the entrance of that room to learn how to reach the mountain hermit: jump 10 times at the end of the tunnel to the west.

Exit the castle the way you came in, and go south and talk to the Fortune Teller, then say Yes if you're ready to remove your disguise. After the cutscene, go south to return to the overworld.

Go west along the beach, then enter cave west of Shecola.