After you get Din's Pearl in Dragon Roost Cavern, you go to your next destination: Forest Haven.
Make Some Preparations
Be sure to check the postbox on the island in case you have mail.
Then get back into your boat and cruise around to find the fish guy that is jumping around, and give him bait to get this area marked on your sea chart.
If you don't have any hyoi pears yet, go to Beedle's shop ship and buy some.
Go into the Forest Haven
Go up the ledges near the postbox until you encounter a boko baba. To defeat it, lock onto it, stay back, and press A to do a jumping attack with your sword to make it point up. Then hit the exposed stem. Boko babas drop seeds, which you can use here in the Forest Haven to get blue potion.
From there, go to the right, defeat the boko babas, go up the ledge, defeat the boko baba there, then swing over to the little rock outcropping. You may need to hold the R button to change direction on the rope. Once you land, raise your shield right away to deflect the octorok's rocks. Its rock will bounce off your shield and hit it. Then jump to the next ledge that has grass on it, defeat the boko baba there, and jump to the little island in the middle of the water. Climb the ledge, jump to the next rock, then raise your shield again to defeat another octorok. Then use the grappling hook to go into the little pool where the octorok was, and go inside.
Help the Deku Tree
Go to the right, climb up the waterfall, keep going around and up another waterfall, and you'll reach the Deku Tree. Get close to it, and chuchus will appear all over it. Do a rolling attack into the tree to knock off all of those chuchus, and defeat them all. There might be another one on the tree's face after that, so just do another roll attack and defeat it, then watch the scene.
Catch a Firefly
The Deku Tree will make a leaf appear in its branches for you. Before you go up there, empty a bottle if you don't already have an empty one, then look around in the grassy areas near the walls for a much brighter firefly than the others. It is a Forest Firefly. Catch it in your bottle.
If you want some potion made from your boko baba seeds, go up the hill to the right of the Deku Tree and into the door. Hollo will make potion if you have an empty bottle and 4 seeds.
Get the Deku Leaf
To the left of the Deku Tree, there is a purple flower bud called a baba bud. Go into it and it will shoot you up into the air. If you don't touch the D-stick, the bud will just shoot you straight up, and you'll land back in it again. This is very helpful if you're not sure if you are aimed in the right direction.
After you get into the bud, aim with the D-stick, and when you're facing the desired direction, hold up on the D-stick and the bud will shoot you forward. You can also hold down on the D-stick if you want to go backward, but you shouldn't have to do that.
Whenever you are launched into the air, look for another bud higher up. Jump from bud to bud. Eventually you will get into a bud and won't see another bud nearby. Above you, there is a branch with leaves on it, and you can land on it from the bud. From that branch, use the grappling hook on the branch above, change directions if necessary, and swing to the next bud. Then continue jumping from bud to bud until you reach the Deku Leaf.
To use the Deku Leaf, jump into the air and use it to glide down, but be aware that this consumes magic. If you are outside, be sure to change the wind direction to whichever way you want to go, because otherwise you will go off course. If you are gliding, press the deku leaf button again to bounce the leaf, which can sometimes get rid of nearby enemies. There will be many situations where gliding is necessary to proceed, so don't forget to try it if you are stuck. If you're standing on the ground, use the deku leaf to blow a gust of air. This can disable some enemies and can turn some special switches.
If you are interested in unlocking the figurine gallery, you can take a detour to do so now. Read How to Unlock the Nintendo Gallery for instructions.
Into the Next Dungeon
After you get the Deku Leaf, if you want to skip the Nintendo Gallery sidequest (see above) and proceed to the next dungeon, face the direction of that ledge that has the grass in the shape of an arrow on it, jump off the branch, and glide down to it. Cut the grass there to replenish your magic, and go out.
Cut any grass out here if you still need magic. The Forbidden Forest is too far to reach from this ledge, so face the little island with the mini-cyclone circling it, and change the wind direction to SW. Before you jump, try to wait until that cyclone is between you and the island so that it moves out of the way before you get there, since you don't want to get caught in it.
When you land on the little island, cut grass to replenish magic again, then face the Forbidden Forest and change the wind direction to NW. Before you jump, wait for the cyclone to be to your right. When you see it there, jump forward so you're to the left of it, and start gliding. If you do it correctly, you'll get caught in the updraft. If not, you will lose a quarter heart and end up back on the little island. Wait for the right moment, then jump into the cyclone. As you approach the Forbidden Forest there will be some peahats. If they get too near, tap the deku leaf button to bounce the leaf and get rid of the monsters.
Land at the Forbidden Forest entrance, and cut grass to replenish magic. If the peahats approach, first use your grappling hook on them to get golden feathers, then use your Deku Leaf on them while standing on the ground to blow a gust of wind and make them fall. If they fall onto your platform, you can defeat them with your sword. Then go into the Forbidden Forest.