The map you found in the Temple of the Ocean King has a special mark on the Isle of Ember, so you should sail there.
Sailing to Isle of Ember
You can tap the feather pen on the map to draw the route that you want the ship to take. You can draw a line to any anchor icon to go to that destination. You can also draw a line to the icon of Beedle's face to stop at his shop.
On the way to the Isle of Ember, you can stop at Cannon Isle, but there is nothing to do there right now except meet the postman.
As you go along, a barrier will come up from underwater. You can tap the up arrow in the bottom middle of the screen to make the ship jump over it.
Isle of Ember
When you reach the Isle of Ember, Linebeck will stay with the ship.
The house close to the dock is empty, so go east. The sign next to the stairs says that the Fortune-Teller is up the stairs, so go up and into the house with the decorations above the door. It seems to be empty, so go down the stairs in the house.
You will be locked in the room, and some Octoroks will appear. Kill them all, and the exit will open. Before you go, check on the sign next to the door. Ciela will tell you to call out something so the person on the other side of the door can hear. You can just blow air into the microphone if you want.
Astrid says that the map next to the door is the key to opening the door, but only Astrid's assistant Kayo knows how to open it. If you look at the map, it says to mark the locations of the island's three torches.
Go outside and go southeast to the house there. Tap the book in the house, and it tells you to go east to pick up the torch. Notice that there is a torch in this house.
There are yellow Chuchus on this island that will hurt you with electricity if you touch them. You have to wait for their electricity to turn off before you try to hit them.
Go to the east side of the island and you will find a skeleton. The ghost will show you the location of one of the torches, so be sure to mark it on your map.
Go counterclockwise around the island from here and you will find another torch at the north side of the island.
Then go back to the basement of Astrid's house and tap the map, then tap the locations of the torches (Kayo's house, the eastern island, and the northern area).
Astrid comes out and asks you some questions. You can answer her however you wish. When she tells your fortune, say Yes! when she asks if you still want to rescue Tetra. You can talk to her again and she will tell you that she unlocked the door leading to the Temple of Fire.
Go east from Astrid's house until you reach the stone door. Tap it, then make your way up the mountain. The volcano will erupt, so stay away from shadows on the ground.
When you reach the temple, you find that the door is sealed. You need to blow out the candles using the microphone. Make Link look at each candle, then blow into the microphone to blow it out. When both candles are out, the door will open. Go in to enter the Temple of Fire.