As you approach the island, the King of Red Lions points it out to you. You have to obtain the second pearl, Farore’s Pearl, from the earth spirit that is located here, the Great Deku Tree. However, it may not be easy, as Ganondorf may have already sent his minions here and harmed the spirit and/or taken the pearl.
Once on the island, check the mailbox for shaking. I mentioned earlier that I probably messed up my notes and that a mailbox here or on Dragon Roost may be shaking. If it is, examine it; you should get a Piece of Heart . If not, you already got it or I was mistaken on the time frame or something.
Anyhow, climb up the ledges. You’ll find a weird red-purple bulb in a patch of grass in front of you. When you get near, it begins to be more of a demonic Piranha Plant from the Super Mario games. It even acts like one, being able to eat you! However, it is not a Piranha Plant, but a Boko Baba Plant.
Once there, look up. You’ll see a stick you can use the Grappling Hook on. Swing across the island near the waterfall. The river has a current, so try to stay out of the water. Now, you may notice an octopus-like thing called a River Octorok. It shoots weird balls at you. Defend with your shield to bounce them back and kill the monster.
Continue to the next ledge; you’ll find another Boko Baba Plant. Defeat it and cross to the next ledge on the river. Far across, in the calm pool of water, is another River Octorok. Defeat it and look up. You’ll see another stick you can use the Grappling Hook on. Swing across and enter the large tree - the Forest Haven.
Inside, run along the shallow river and climb the ledges to its higher parts. Climb onto land after the second ledge and approach the large tree from the side that looks like it has a face. As you do, you’ll notice it has a bad case of acne - not the typical kind, but Chuchus! Use the A button to roll into the tree to knock all of the Red & Green Chuchus off. Then you can attack and kill them.
Linder then reminds the Great Deku Tree that Link cannot fly like the Koroks can - he is too heavy. The Great Deku Tree then creates a special (?) leaf, a Deku Leaf, that you can use to fly for a short time. He conveniently puts it near the top of himself (you’ll see why it is convenient in a bit).
Begin by going forward and left, towards the bulb that looks like a Boko Baba Plant, but is more of a purplish-blue color - this is a Boko Baba Bud. You can use these to jump a short distance into the air and forward; they also restore part of your magic power, which will be bestowed upon you shortly. Walk next to the Boko Baba Bud and walk "into" it. Link will jump in.
After three or so launches, you’ll end up on a leafy platform. Walk forward towards the edge. Look up and you’ll see a stick you can use the Grappling Hook on. Use the Grappling Hook and swing towards the next Boko Baba Bud, pressing A when near.
Repeat the previous process and you’ll get to the ledge with the Deku Leaf on it. This allows you glide through the air! You can also use it on the ground to blow gusts of air that can move things and stun enemies.
In the air, you still can stun enemies - just use the button the item is set to. You are also given a Magic Meter, which displays how much magic you have left (sort of like a Final Fantasy MP Gauge) like the hearts display how much health you have (like a FF HP Gauge). Magic is the power used to determine how long you fly, among a few other things.
Below you, you’ll find a ledge with some grass in the shape of an arrow. Set the Deku Leaf to X, Y, or Z and jump from the leafy platform. In the air, press the button you set the Deku Leaf to to begin gliding. Glide to this ledge. You should cut the grass here to replenish your magic power - you’ll see these items as pots and bottles of sorts. When ready, go through the gap in the wall.
There, you will cut the grass to restore your magic, then switch the wind to the north-west. It soon becomes obvious that the entrance to the Forbidden Forest is even higher now. You’ll have to fly through a whirlwind that is circling this small island. Wait for it to enter your field of vision, then jump on the ledge and fly. You may go through it; if not, you’ll be returned to the place your previously touched ground at (minus 1/4 heart).
Once you manage to make it into the whirlwind, begin to fly towards the thorn-covered island. Shift a bit to the right to make it to the entrance. Along the way, you’ll meet some flying enemies called Peahats. At the moment, just constantly press the Deku Leaf’s button as they approach; maybe they’ll leave you alone. Once you get to the entrance, enter your second dungeon.