After you visit the Forest Sanctuary, you can enter the Forest Temple.
Learn the Song of Healing
When you reach the train platform of the Forest Temple, go north, but don't go through the door yet. There is a rocket ship to the left of the door. Tap it to learn the notes that you should play, then play it on the Spirit Flute. The song is Green, White, Green. It's the Song of Healing, and you can use it to refill all of your hearts in a dungeon. You can only use it once per dungeon, though.
Enter the Temple
Now you can go through the door in the back wall. There is a locked door ahead. Go around and you will eventually come across some nuts in the ground. Pick one up and go to the right and throw the nut at the switch across the way. A bridge will appear. Cross it and then go south.
Eventually you will be locked in a room with some spinuts. Kill the spinuts and a chest will appear, and the two doors in the room will unlock. You can't do anything else on this floor, so go upstairs.
There are toxic purple gas clouds up here, so stay away from those. Go north and you will be locked in another room. The enemies in here are Vengas. They will self-destruct after you hit them, leaving behind some toxic gas. Avoid the gas after you defeat the bugs and go open the treasure chest. Inside is the Whirlwind.
To use the Whirlwind, tap the Whirlwind icon in the top-right corner of the screen, then drag the stylus to change where Link is aiming. Then blow into the microphone to send a gust of wind in the direction that Link is facing.
Go south and use the Whirlwind to get rid of the toxic gas that you passed by earlier. You will reveal a chest that contains a random treasure item. Now go back to the room where you got the Whirlwind, and point the Whirlwind at the propeller thing at the top of the screen. This will unlock the door to the left.
You will find a statue that you can awaken by playing the Song of Awakening (blue, orange.) This statue will tell you if there is any hidden treasure on this floor. There are other statues like that on other floors of temples around the world.
Use the Whirlwind to blow away the purple clouds blocking the stairs, then go up.
You will find a key sitting on a platform in the middle of the room. Use the Whirlwind to blow it onto the part of the floor that you can reach. Then step on the switch to the right and go through the door. This is the area where you were before, but now it's infested with Vengas.
Go to the top-right area and blow all of the purple clouds away. There is a stamp station in the top-left part of this area, but you can't use it because you don't have a stamp book. We'll get back to that later. There is also a switch next to the stamp station. Step on it to make a treasure chest appear.
Go back across the bridge and open the treasure chest. It contains a big green rupee.
Now go to the locked door in the middle of the room and tap it to unlock it. Beyond the door, there is another statue that will tell you if there are any treasures left in this room.
If you go a little farther, you will be locked in a room with some Bubbles. They are surrounded by a purple cloud, so to stun them, you need to use the Whirlwind on them and then hit them with your sword. When you do, the doors in this room will open and you can go upstairs.
In this room, there is a locked door to the right, and there are Vengas around. Just go north to the stairs. In this area, there is a nut and a switch. Use the Whirlwind to blow the nut into the switch. Open the chest for a small key, then go back downstairs and to the right and unlock the door.
You will be locked into this area and you must fight a Mothula.
Wait for Mothula to spit out a Bubble, then use the Whirlwind to shoot the Bubble back at it. While it is stunned, hit it with your sword. Be careful not to blow a gust of wind at the Mothula if there isn't a Bubble, because the Mothula will just blow the wind back at you and stun you before swooping in to attack.
When the Mothula is gone, go upstairs. Go south and you will pass by the gate that will only open with the Big Key. Continue going south and you will find some blastworms to the right. When you hit a blastworm, it curls up into a spiky ball. You can then use the Whirlwind to blow the ball at things, making it blow up. You can use this like a bomb. Use the blastworm to blow up the blocks at the right side of the room.
Go past where the blocks were and use the Whirlwind to blow the purple clouds away from the winding path. Be careful not to fall into the pit. At the top of this area there is a statue that you can awaken with the Song of Awakening. There is also a crystal switch. Hit it and the door in the bottom-left area will open.
Go through the door and you will see that a crystal switch is at the top of this area, but there are a bunch of blocks in the way. Hit a blastworm and use the Whirlwind to blow it over to the blocks to break them apart. More blastworms will randomly fall from the ceiling, so if you run out of them, just wait for more to fall. After breaking the blocks, use another blastworm to hit the crystal switch. Once you hit it, the door in the top-right corner of this floor will open, giving you access to the Big Key.
Go to the room with the Big Key, but before you pick it up, check the map in the back of the room. Tap the Map button to switch to your own map and draw the arrow and skulls onto it. You will need to follow the path of the arrow and avoid the skulls after you pick up the Big Key and walk to the gate with it, because otherwise you will encounter a trap.
Take the Big Key and walk with it along the path that you saw on the map behind the Big Key before you picked it up. When you get to the gate, tap on it to unlock it with the Big Key. Then go upstairs.
Break the pots to refill hearts, and read the sign to make a glowing blue light appear that you can use to warp back to this room from the entrance. When you are ready to fight the boss, go upstairs. The boss of this temple is Stagnox.