Now that you have the Titan's Mitt, you can get a lot of great upgrades.
Go south of the Village of Outcasts to where the Library is in the Light World, and you find a frog-creature trapped behind some black skulls. You can lift those now, so do that and talk to the frog to have it follow you. He begs you to take him back to his partner in the other world. Use your Magic Mirror to return to the Light World, and the frog-man turns into a human (a tiny old man, to be precise).
You probably haven't visited the Smithy in Kakariko, but if you have, you learned that one of the smiths had gone missing. Well, you've found him! Go to the Smithy, which is reached from the south-east exit of Kakariko. Inside, the smith will return to the anvil with his partner. Go outside and back in and talk to one of the smiths, who will offer to temper the Master Sword for a mere 10 rupees; a real bargain! Take the generous offer and go outside without your sword. You are somewhat defenseless now, so don't wander too far (remember; there are soldiers patrolling Kakariko Village now). You can use your hookshot and things to defend yourself. You can freeze enemies with the ice rod, then shatter them with your hammer, revealing magic jars! Pretty slick!
You can pass the time by getting another upgrade right here by the Smithy. You won't need your sword for this. Use your hammer on the post to the right of the Smithy, then jump off the ledge to fall into the hole, where you find a cave. Go through the north exit to find a creepy sort of sculpture. Sprinkle some magic powder on the red thing in the center and a purple bat will come flying out of it. It will curse you for disturbing its slumber, then it will change its mind and thank you..? It will talk about getting revenge... if that's OK with you. It will then send down tiny lightning bolts that encircle you. The bat says this curse will reduce your magic power by half! Uh oh... Sounds bad, but in reality, the rather confused bat did you a favor. Now you will only require half the magic you once needed for your magic items!
Exit south, then west, and up the stairs and outside. Your sword is probably not ready yet, so wander around town for a bit (talk to the locals, knock back a few beers, etc.), then go back into the Smithy, where (if you waited long enough) your Tempered Sword should be ready for you! It has an orange blade... snazzy!
Go back to the warp you left near the Library to return to the Dark World. Go where the Smithy is in the Light World and lift one of the black boulders. Before venturing up into the ruined house (which is the Gossip Shop you might have heard a fortune teller talk about), notice a bunch of purple posts in the ground. They are pretty fun to pound into the ground, so go around and pound them all down (you may hit some of the hapless, creepy skeletor-cuccos; just be sure not to hit them too often). At the bottom of the screen, stairs will appear. Go down them to where a piece of heart is sitting right there for you to take!
Now go back outside and walk north to the Gossip Shop, which has a chest inside. When you walk up to it, you'll discover that the key is locked inside so you can't open it. You'll just have to take it with you. If you dash with A, you'll leave the chest behind, so try not to lose it.
Remember hearing about an ex-thief who used to pick locks, who is now hiding out in the desert? Let's look for him... Walk away from the house and use your Magic Mirror to go back to the Light World. The chest will tag along with you. Use the Flute to go to #7, the swamp. The chest will still follow you. Go west from here towards the entrance to the desert. Along the way you will find a man sitting beside a sign. When you approach him, he admits he is aware that you know about his thieving past. He will open the chest for you if you promise not to tell anyone about his past. So make the promise and he opens the chest, which contains the fourth bottle! I would suggest that since you should already have three fairies in your bottles (which should be more than enough for boss battles), you should go to the Waterfall of Wishing by Zora's Domain and throw in your empty bottle. The fairy will fill it with green (magic-refilling) potion for free! You should know how to get there: use your flute to reach #2, the witch's hut, and head towards Zora's domain, then go into the waterfall.
We are actually going to use this magic potion now to get a new item, then get a refill after we're done.
Use the flute to fly to #1, Death Mountain. Climb to the top where the Dark World warp is, and step on it. From here, go straight down and drop off the ledge, and you land on a small ledge with a door. Go inside. This room is filled with spikes, so how are you going to get through? I'll tell you how: the Magic Cape! Pound one of the pink things down with your hammer (or all three to make life easier), then don the cape to become invisible, and walk along the spiked path (you can dash if you want; might save magic), then lift the heavy block and you reach a chest. It contains the Cane of Byrna. Not really a very useful item, per se; when you use it, a circle of energy will spin around you, protecting you from attack, until you stop it with Y. It kind of does the same thing as the Magic Cape, making you invulnerable (but not invisible). It also uses a lot more magic than the Cape, so there's not much point to it; we're just getting it for completeness' sake.
The skulls around the sides just hide hearts; would be better if they held magic jars, but oh well, we have green potion. Use the green potion now and get back across the spikes and outside. Use the Magic Mirror to get back to the Light World, and play the flute and go to #2, the witch's hut again. Go back to the Waterfall of Wishing and get yourself a refill on the green potion. You can just use it now to refill your magic, then throw the bottle back in for yet another refill. You'll need it against the next boss.
Okay; once outside the Waterfall of Wishing, use your flute to go to Kakariko Village. You need to start in the Light World to reach the next dungeon. From Kakariko, take the second north exit from the left, which puts you in front of a large black boulder. Pick it up, then pick up the white stone in here to reveal a warp to the Dark World! Step on it.
Once in the Dark World, go straight north to find the Skull Forest. Follow the path, careful not to let the pikit eat your shield or take other items from you, and continue east past the skulls. At the fork, go north, and you find a hole. It looks dangerous, but it actually leads to the next dungeon, so drop down to find yourself in...