As the King of Red Lions tells you, play the Wind's Requiem to choose south as your next direction, then embark and unfold your sail. After a few meters, a fish will appear. He will talk about sea charts and suggests you feed fishes like him as you meet them. Keep on sailing south, until you reach the next big island.
Climb the hill while getting rid of the nasty BokoBabas on your way, and pick up their Bokobaba Seeds. Use the Grappling Hook to jump on the small island, then go left, then right. Raise your shield to deflect the rock back at the Octorok, then use the Grappling Hook again and jump in the shallow waters. Take the passage on the right.
Go ahead to the right, walk against the stream, then go left. Walk around the place until you reach a water lily, then the giant tree will come to life and get attacked by Chuchus. Go left of the tree, perform a roll attack on it so the Chuchus fall on the ground and attack them. The Deku Tree, the Earth Spirit, will then tell you about the Korok people and about Makar who got lost within the Forbidden Woods. To help you in this new quest, the tree grows a leaf. Use the purple flowers, called Baba Buds, to jump and climb to the leafy branch halfway up. Then use the Grappling Hook to reach the next flower, and so on until you reach the top of the tree. Pick up the Deku Leaf and you will receive a magic meter as well. Thanks to this Deku Leaf, fly towards the Korok, talk to him, then go down the passage. Fill up your magic meter with the bottles you'll find by cutting the bushes, and talk to another Korok. Play the Wind's Requiem, have the wind blow south-west, then use your Deku Leaf to fly towards the small island to the left. Speak to the Korok about wind currents, then change the wind's direction for the north-west. When the current get close, jump using your Leaf to go upwards, and fly towards the next island. Go into the passage.