After you get your sail in Windfall, you can go to Dragon Roost Island.
Get the Wind Waker
Your boat gives you the Wind Waker and teaches you how to use it. Play notes by pressing up, down, left, right, or center on the C-stick. When the metronome at the top of the screen hits the red circle in the center, the note plays. If you don't move the D-stick, you'll play in 3/4 time. If you press left on the D-stick, you get 4/4 time. If you press right, you get 6/4 time. If you have trouble getting Link to move the baton, let the baton go back to center position before moving on to the next note. If you mess up a song and want to start over, just change to a different time scale with the D-stick to reset the song.
Learn the Wind's Requiem
Now it's time to learn a Wind Waker song. Walk through the small cave that should be a little to the right, at beach level. You'll see a small island not too far from shore. Swim to it, and use your Wind Waker in front of the non-broken slab. You'll learn the Wind's Requiem.
Reach the Rito Town
Now swim back to shore and head back through that cave passageway. Turn right to find some bomb plants. Use the bomb plants on the rocks that are blocking your way. At one point you'll have to bomb a rock by throwing the bomb up on a ledge, which is somewhat tricky. Sidle across when you have to. Then when you drop down you'll find a rock with two blocks on top of it. Bomb that rock, then jump down to where the blocks fell. Pull the bottom block out until the top one falls. Climb up onto the first block by pressing A while walking up to the rock. Then climb up onto the ledge to the right. Keep going and you'll talk to Quill. Follow the path and eventually you'll reach the Rito town.
After the scene, you receive the delivery bag. Go up the ramp at the side of the room, and go in the first room to the left. Talk to the girl, named Medli, and you'll get the letter to Komali.
Play the Mail Sorting Game
Exit the room and turn left. Keep going until you reach a desk with a Rito sorting letters. This is the mail sorting game. The object of the game is to match the symbol on the letter at the bottom of the screen to the symbols in the mail slots in 30 seconds. The key to winning is to just not be too hasty, because if you make a mistake, you'll lose a lot of time. First you need to sort 10 letters, then 20, then 25. Keep playing until you've successfully made it to 25 letters. Then leave out the door to the left of the sorting desk, and come back in. Talk to the new guy at the mail sorting desk and he'll want you to get 25 again. Try until you get it, and he'll ask you to do something for him. Agree to do it, then take his letter to the postbox around the front of the island (you saw it on your way up to the town). First select the delivery bag, then equip his letter to Y, Z, or X, then stand in front of the postbox and use the letter. Pay the postage.
Give the Letter to Komali
Go back into town, and head straight across the bottom floor to a passageway that leads to Komali's room. Give him his father's letter. Go back out.
Get into Dragon Roost Cavern
Out of the passageway, turn left and head through the door with the feathers hanging from it. Outside, go down to where the bridge is out, and climb down below. Talk to Medli. Pick her up, then stand on the little hill behind you and press L to face the other side where Medli wants to go. When the wind is blowing toward that side, throw Medli. If she doesn't make it, just try again.
You receive a bottle. Use the bottle in the little area of water nearby. Climb up the piece of bridge that you first came down from, and walk over to those red plants. Stand near a plant, then use the water on it and it will turn into a bomb flower. Pick up the bomb flower and throw it over the ledge. Try to get it to land on the rock. The rock will explode and the spring will fill up the area below.
Go into the water and swim across to where you threw Medli, and climb up. Careful of the lava here. Throw a bomb flower into the pot to the right to make the statue fall. Next, pick up a bomb flower and stand on the fallen statue, and throw the bomb into the next pot. Once that is done, follow the path into the dungeon.