This part will take you through the entire Forest Temple.
Now that we're at the entrance of the Forest Temple, head to the upper left and you'll see a rocket-shaped statue humming out a color-coded tune. Use the Spirit Flute to learn the tune. It is the Song of Healing, and it can summon a fairy to appear within temples or the Tower of Spirits to heal Link completely. This tune can only be used once per temple.
Inside the Temple
Go north until you reach a locked door, then go to the right and up where you see some nuts growing on the left side and a switch orb across a gap on the right side. Pick up a nut and carry it close enough so you can throw it across the gap to hit the switch orb. This will cause a bridge to appear. Use the bridge to cross the gap and travel to the right, down, left, and down until you enter a room where the door closes behind you. Then a few Spinuts will pop up which you must defeat. This will make the door reopen and a treasure chest will appear in the upper left corner of the room, which will give you 100 rupees. Another door will open in the lower right corner of the room. Go through that door and up the stairs to get to Floor 2.
Into the room you enter, go north and avoid the clouds of purple smoke that are in your way, then enter the room north of you whose door will close behind you. Then four Vengas beetles will appear. Slay each of them, avoiding the purple clouds they leave behind, and a treasure chest will appear in the upper right corner while the door reopens. Open it, and you'll get the Whirlwind, a device that will be very useful for getting through this Temple. To use the Whirlwind, aim it in the direction you want to blow it in and then blow into the microphone. Use the Whirlwind to make a propeller pillar at the top of the room move, and it will cause a door to open in the upper left corner. Go through that door and enter the room where you'll need to blow away purple smoke that blocks the door to the staircase on the left. Go through that door and down the stairs to Floor 1.
Go down and to the right in the room that you enter, standing south of a small key that stands on a platform in the middle of a square gap. Use the Whirlwind to blow the key to the north side of the room, grab it, and then go to the upper right side of the room and step on the floor button to open a door. Then go down and back into the room with the nuts and the switch orb, go down and then to the left where the locked door is, then open it with the key. Then go down the hallway to the left, then down, then right, and then down into a room where the door closes behind you. This time you'll have some Bubbles floating by with purple clouds surrounding them. Use the Whirlwind to knock the Bubbles onto the ground and then destroy them with the sword. The door will reopen, and another door in the lower left corner of the room will open. Go through that door and up the stairs to Floor 2 again.
Go right in this room and then north to the door with the stairs leading up to Floor 3. In the room you enter on Floor 3, you'll see a switch orb across a big gap and a nut standing lined up below it on the other side that you're on. Use the Whirlwind to blow the nut across the gap and hit the switch. This will make a treasure chest appear, in which you'll find a small key. Take it with you back down to Floor 2 and use it to open the locked door that leads to a room in the center of that floor. Enter that room, and the door will close behind you, and you'll be facing the Temple's mini-boss, Mothula.
Mothula will appear and then disappear if Link gets too close to him. When he appears at a distance, he will launch Bubbles at you. Use the Whirlwind to blow the Bubbles back at Mothula, stunning him so you can get close and strike him. Repeat this a few more times and Mothula will be done for. The door to the south will reopen, and a door to the north will also open. Take the north door and go up the stairs again to Floor 3.
In the room on the upper right section of this floor is the Big Key we need to get to the boss, but we need to find a way to open the door to this room. Go down to the room in the south and strike the Blastworm to make it puff up into a ball, then use the Whirlwind to knock it into the blocks between you and the room toward your right to blow them up. Inside the next room, use the Whirlwind to blow away the purple smoke covering up a narrow path that leads to a switch orb we need to hit. This will cause a door to open that will allow us to enter the room in the lower left section of this floor. Go back to the left and enter that room, where the trick is to use puffed-up Blastworms to blow up blocks across a big gap that prevents us from reaching a switch orb with a Blastworm. Once the switch orb is hit, it opens up the door to the room where the Big Key is. Go to that room and pick up the Big Key, but to get it the door that it opens we need to stay away from the tiled floors, for this will summon Key Masters to appear which will grab the key from us and put it back into the room we got it from. This requires going around in a counter-clockwise circle to get to the door with the Big Key. Use the map that's located in the room where the Big Key is to figure out which way you need to go. Once the door is opened, go up the stairs to Floor 4.
There's really nothing we need to get in this room, just some pots for us to break to get rupees and hearts, and a blue portal that will take us back to the entrance of the Temple. Go north and up the stairs to meet the boss of the Temple.
Temple Boss: Stagnox
Now we'll be fighting the Temple boss Stagnox. In the first part of the battle, Stagnox remains on the ground and will turn to fix his gaze on Link until he stops and lunges at him. Try to get behind Stagnox where his behind is covered with purple smoke, then blow away the smoke using the Whirlwind and strike his behind. After doing this a few times, Stagnox will take to the air and drop Blastworms onto the ground, hovering in the air for a while before he divebombs on you. Strike one of the Blastworms so that it bloats up into a spiky ball, then use the Whirlwind to blow the Blastworm toward Stagnox during one of his divebomb attacks. He will fall to the ground, and Link can hit his behind during that time. After a few successful hits, Stagnox will fall and go poof, and a Force Gem statue will appear, indicating that you've completed this temple. Collect the Heart Container that is inside the treasure chest in the room and then step into the blue portal to return to the Temple entrance.