After you rescue BowWow from the Moblin Hideout, you can take him to Goponga Swamp, where he can eat the flowers blocking the path to Bottle Grotto.
Get a Key
In the first room, there is a treasure chest, but it's blocked by pots, so just go north for now.
In this dark room, use Magic Powder to light the two torches in the center. This opens the door to the east. Go through.
Kill the Stalfos. A key will fall from the ceiling. Take it.
Get the Stone Beak
Go back to the room where you lit up the torches. Go west through the locked door.
Carefully avoid the Traps that slide toward you, then kill the Keese in this room, lighting the torches if necessary.
When the Keese are gone, the north door opens. Avoid the Traps and go through.
Go to the left or right side of the room to make the Hardhat Beetle go to the side, then run to the middle and jump over the gap. Open the chest to get the Stone Beak. While still on the north side, use your sword to knock the Hardhat Beetle into the gap. This makes the door open.
Get the Compass
Go back to the room with the two Stalfos where you got a key. Go south through the locked door. Hold the sword button, then walk past the Shy Guy so that its back is facing you, then let go of the sword button to swing it in a circle, hitting the Shy Guy's back. When you kill it, a chest appears. Open it to get the Compass.
Get a Second Key
Go north from the compass room, then go east and hit the orb, then go south.
Hit the orb that is inside of the blocked area to lower the blocks. Open the chest to get a key.
Get a Third Key
Make sure the blocks are down (hit the orb if necessary) and then go east.
Jump across the gaps, jumping to get the flying Magic Powder, then step on the button to make a chest appear. Open it to get another key.
Get a Fourth Key
Go north to the next room. Hold the sword button and walk past the Shy Guys so that their backs are toward you, then let go of the sword button to swing it in a circle. You will kill the Shy Guys, and a key will fall from the ceiling. Go south, then west, then north (if the blocks are in your way, go back and hit the orb), then east to get the key that just fell.
Reach the Mini-Boss
Go back to the east side of the room where you killed the Shy Guys, and go east. Open the locked door and go through.
There are two block in the middle of the room. Push them toward the center so they cover the blue tiles. This makes some stairs appear. Go down the stairs.
In the underground, jump onto the moving platform and ride it to the left side of the room, then exit west. In the next room, jump over the spikes, then jump onto the moving platform and ride it to the top, then climb the ladder up to the next room.
In this dark room, just go through the one-way door in the north wall.
Keep your distance from Hinox. If he catches you, he will throw you and do a lot of damage. There are also some cracks in the floor near the walls, and if he throws you onto one, you'll get damaged even more when you fall through. Rush in and hit Hinox with your sword when you can, but run away after you hit him because he will throw a bomb afterward.
Get the Map
After you defeat Hinox, a fairy appears, and a warp spot appears. The warp spot takes you back to the entrance of the dungeon.
Use the east exit to leave Hinox's room.
Jump over the gaps and use the northeast exit. Be careful of the Keese hiding on the statues.
In this room, there is a Vaccuum Mouth in the corner that sometimes pulls you toward the corner. Carefully go to the chest in the northwest corner of the room and open it to get the map.
Get 20 Rupees
Exit north from the room where you got the map.
Jump to get the flying Magic Powder. Open the chest in this room to get 20 rupees.
Get the Power Bracelet
Go west through the locked door.
Equip the Magic Powder. If the ghosts get too close, just hit them with your sword. Use the Magic Powder on a torch. When the room is lit up, the ghosts turn into shadows and are vulnerable to your sword.
After you have killed the ghosts, open the chest that appears. It contains the Power Bracelet. When you have it equipped, you can use it to lift pots, rocks, and other heavy objects. To use it, touch a pot or rock, then hold the button that you assigned the Power Bracelet to, then press the direction away from the object that you are touching. You will lift it. Press the button again to throw the object.
Get a Fifth Key
In the previous room, lift the pots using the Power Bracelet. Use the northeast exit.
Walk over and hit the orb. If you can't reach it, go to the south part of the room and throw a pot at the orb, then go to the north side of the room. Hit the orb a second time to reach the chest in the northeast part of the room. Open it to get a key.
Get the Nightmare's Key
Hit the orb while standing on the blocks to the right of the orb. Walk east on top of the blocks and then exit northeast.
Go east, being careful of the red Zol and the Hardhat Beetle. Try to lure the Hardhat Beetle to the left side of the hole, then hit it with your sword to knock it into the hole.
In the next room, use the south exit. In this room, there is a Shrouded Stalfos, a Pols Voice, and a Keese. The map tells you that there is a hidden chest in here. However, it's not enough to just defeat the monsters in this room. If you use the stairs in this room, you will reach a room at the other end that tells you to defeat the monsters in a particular order. First, you have to defeat the Pols Voice, then the Keese, then the Stalfos.
To defeat the Pols Voice, push an upper block down and the other upper block to the side, then pick up a pot and throw it at the Pols Voice. Then kill the Keese, then the Shrouded Stalfos. If you did it in the correct order, a chest will appear. It contains the Nightmare's Key.
Reach the Nightmare
From the room where you got the Nightmare's Key, go north, then unlock the door in the east wall and go through.
Lift pots and throw them at the Pols Voices to kill them, then kill the Red Zol. After all the enemies are gone, stairs appear. Go down.
Stand on the stone platform to make it go down. Then exit to the west. In the next room, lift the pot and carry it over to the stone platform. With the extra weight of the pot, the platform will go down. Go up the ladder.
Jump across the gaps to reach the Nightmare's Door. Unlock it and go through.
Wait for the Genie to start throwing flames. After he throws them all, he will go back into the bottle and start hopping around. Hit the bottle with your sword to stun him, then use the Power Bracelet to pick up the bottle. Throw it at the wall to hurt it.
Repeat this process until the bottle breaks. Then hit the Genie with your sword. The Genie will become temporarily invulnerable. Then it will reappear in one of the two locations that it split to. Continue hitting the Genie with your sword, avoiding the fireballs that it throws, until the Genie is defeated. If you are having trouble avoiding the fireballs, try standing in the middle of the south wall while the Genie spins, then walk to the left or right when the Genie reappears. I find it easier to avoid the fireball this way.
Get the Musical Instrument
After you hit the Genie enough times, it dies, leaving a Heart Container. Take it, then go west to the next room. Go up to the Musical Instrument to get the Conch Horn.
After you get the instrument, you see a hint about going to the Prairie. This is a reference to Ukuku Prairie, which is east of the village where Marin and Tarin live.