Now that you have all five elements, you should return to Hyrule Castle to complete the Picori Sword.
Return to the Elemental Sanctuary
You'll be atop the house in the clouds, and now your destination is the Elemental Sanctuary. But first! There are a few fusions you can do now. Once you infuse your sword with the last element, you have to go ahead and start the final dungeon; however, it is possible, although a little tricky, to get out of the dungeon once it's started. I'll have you do that later. You need to revive the Four Sword to be able to complete all the fusions in the game, but you need to get out of the final dungeon to perform them. Plus, if you don't leave the final dungeon after it's started, you won't be able to boost all your abilities and get all your pieces of heart before tackling it.
So, for now, let's do some fusions that opened up after beating the Palace of Winds.
You can't use your ocarina here on the roof so go down through the house and out the front door. Use the ocarina to warp to Lake Hylia.
Walk up and around to where David Jr. is sitting, and jump onto the platform to the right. Keep jumping from platform to platform, get the piece of heart, and get to the end where you can use the mole mitts. Dig your way in.
This is quite a maze, and there is plenty of treasure to be had. To the southwest, there are a total of three hidden chests. The rightmost contains a red Kinstone piece, the bottom left a blue Kinstone piece, and the top one 50 mysterious shells. Continue west.
Beat the moldorm and then dig away the dirt from the far wall to reveal a symbol. Fuse with it to make the final Goron appear at Goron Cave.
Go back to where all the paths split out, and take the path to the right. Though there are many holes here, there is only one hidden chest, so find it for a red Kinstone piece. Go up the ladder above and once outside go down the ladder to the left. Dig to the next ladder and go up. You'll reach the beanstalk you caused to grow with an earlier fusion. Climb up to get (left) 200 rupees, a piece of heart, and (right) 200 mysterious shells. Now go back into the cave, beyond the ladders and to the area where all the paths split out again.
Now take the long path west. When the path splits, go left and up first and reveal two hidden chests, the left containing a blue Kinstone piece, the right a red piece. Keep digging east a long way to a chest containing 50 mysterious shells. Go back the way you came, and at the split dig down to an exit. Grab the piece of heart. To make your life easier, just warp to the Lake Hylia Wind Crest. You want to reach Waveblade's dojo, he has a Kinstone to fuse.
Now fly to Mt. Crenel.
Make your way to Grayblade's dojo and fuse Kinstones with him.
Now fly to Hyrule Town, and make your way north to Hyrule Castle Garden.
Hyrule Castle Garden
Go to the southeast corner and down the ladder to Grimblade's dojo and fuse Kinstones with him.
And that's all the fusions you can do before reviving the Four Sword.
So, now, sneak back into the castle past the guards as you did before, make your way to the sanctuary, and infuse the blade. You can now split into four, and if you charge up your sword and do a spin attack, the blade will shoot out a spinning energy beam.
Go up through the door that appears and check the stained glass windows to learn the true nature of the light force. Unfortunately, Vaati is there to discover the secret too. (The thing that I find strange is the fact that at the very beginning of the game, just before he turned Zelda to stone, Vaati spoke of the power of light that flowed through her veins... why didn't he know about the light force then?)
Anyway, exit south, splitting into four to step on the switches, and continue south. Now you discover that the castle is different. There are people around turned to stone. Use the new spin attack energy beam on each statue to revive him. When you can, exit south.
Dark Hyrule CastleThis is the final dungeon, but you'll want to escape it for a while so you can get all your pieces of heart and items, and boost your skills to the limit (a few pieces of heart only unlock after you revive the Four Sword). Here's how to get out.
Exit west, then north. Carefully avoid the spinning fireballs and continue north. Boomerang the skull enemies to get rid of them. Bomb the east wall, go through and shrink. Go back out the way you came in, and make your way to the spinning fireball room. Carefully go towards the big hole where the four lines of fire are spinning, and drop down. Continue north, up and right through the small arch. In the next room, grow to normal size, press the switch to open the jail cells, and go in and revive King Daltus with the Four Sword. Talk to him and he'll give you a small key. Exit west, then go north up the stairs.
Don't unlock this door (it may be a fake anyway), instead head down to the spinning fireball screen, split into four (with the real Link in the bottom row) and push down the block, and continue south. Go all the way south, ignore the locked door to the left (it's a fake) and unlock the south door and go through. Follow the long path, avoiding or beating monsters, down some stairs to the chest with the map in it. Go back a little bit and up those stairs.
NOW you can exit south and leave the castle. Warp to Hyrule Town.
Now, you CAN try to collect your remaining figurines, however, take note, there are 136 total figurines you can win, but the last 6 can only be obtained after beating the game. BUT, if you collect the first 130 that are available prior to beating the final dungeon, Carlov will consider your collection complete, give you the Carlov Medal, and if you show it to the man sitting in Mama's Cafe he will unlock the back door of the house with the robot on the roof, where you can access a piece of heart, 600 rupees, and an old-style phonograph that lets you use the game's sound test.
There are two drawbacks to winning the Carlov Medal before beating the game: the sound test won't be able to play some of the game's songs (final dungeon and final boss battles and ending credits, for example), and you won't be able to collect all 136 figurines in the game. However, the drawback to waiting until you beat the game to collect all 136 figurines is the fact that you will be short one piece of heart, and therefore won't have all 20 heart containers for the final dungeon and final battles. Since that doesn't make a HUGE difference, I say to wait until you beat the game before going for the rest of the figurines.
Go straight to Swiftblade and learn his final technique. Now if you want you can get all the rest of the issues of his newsletter at the post office. You should probably save the cash for better stuff though.
Fuse Kinstones with the purple-haired teacher in the school. She won't always be willing to fuse, you may have to go in and out of the school multiple times before she is ready to fuse.
You have 7 Tiger Scrolls now, so you can now train with Swiftblade the First in Castor Wilds. Go there.
Dash across the swamp to the left and up to reach the tombstone leading to Swiftblade the First. Train with him to learn the eighth and final technique.
There should be an opening in a waterfall in the very northeast corner of Castor Wilds. Make your way to the small watery area at the northeast of the Wilds. First go on land and through the door to a piece of heart behind a bunch of blocks. The only way to reach the piece of heart is to push the rightmost blocks. Push the top and bottom ones inward, and the middle one up or down. The other blocks won't budge sideways. Grab the piece of heart, then go back out and swim into the waterfall. Train with Scarblade. After you do, you'll be able to charge up your sword faster, and will be able to do more spins with the Great Spin Attack you just learned.
You're done here, and if you did everything here earlier, you should never have to come back! Warp out of here to Hyrule Town, then go north to North Hyrule Field.
North Hyrule Field
Get in the water to the west and swim north (you may have to start above the bridge if you can't swim under it). Go into the waterfall. Train with Greatblade to improve your Great Spin Attack. Go back outside.
Since you're here, you might as well continue north and west to the Royal Valley, where some fusion prizes await you (unfortunately you have to go down the ladder and push the block out of the way again). Make your way into the Lost Woods, and take the easy path left, left, left, up, up, up, up, to get to the graveyard. Past the graveyard gate, there is a joy butterfly flying around the northwest area that makes you swim faster. The two chests, northwest and northeast, both hold a red Kinstone piece.
Warp to Veil Falls.
There might be a golden tektite around, kill it or avoid it and get to the cave opening up above. Go down the stairs and out down below to the water, swim up into the waterfall. Get the piece of heart and go back out. Get back on land and now warp to Lake Hylia, and from there get to Lon Lon Ranch. Find the place near the top where you can use the Cane of Pacci to get up on the ledge (there are two, go to the second one), and go up to the bottom area of Veil Falls. Go up and around and into the water to get into the waterfall and increase the speed of your split gauge with Splitblade.
Outside, swim east and up. The rupees are Like Likes. Up above is a cave, dig into it with the mole mitts. The left chest holds 50 mysterious shells, the right, 50 rupees. Go back out and return to Lon Lon Ranch.
Lon Lon RanchNow you want to go into Goron cave, where the rock walls have finally all been broken down. Fuse with the Goron on the right to make the huge Goron at Veil Falls wake up. You can go back to Biggoron after you beat the game to get the Mirror Shield (I don't get the point of having to wait until AFTER you beat the game to get the shield, as it would only be helpful BEFORE. Oh well.)
Go up the bridge in here to get the fourth and final empty bottle. Now you can leave. Warp to Minish Woods.
Get to the Minish Village. Go towards the abbey, and take the wooden walkway along its right side to get to the water. Dive in and swim to the house to the east. Fuse TWICE with this Minish. That's all the fusions in the game! Now if you go to Tingle in South Hyrule Field, you get the Tingle Trophy, which is useless but you might want it anyway.
Since the new crack at Lake Hylia is easiest to get to, warp there.
Shrink and drop into the crack to find a cave in which you must jump and get into holes to avoid a couple rolling spiky things. Carefully make your way around and climb up the ladder to reach a Minish house. Here you discover none other than Librari, who gives you a FULL heart container (not just a piece of heart)! To avoid having to go through that cave again, maneuver around the crack and hop off the edge into the water below. Swim southeast to the shrinking portal above Mayor Hagen's cabin, and grow.
Now to Veil Falls. Since the Wind Crest puts you in a not-so-good starting spot, you'll want to warp to Hyrule Town, go north to North Hyrule Field, then take the northeast exit to Veil Falls.
Go down into the water and swim to the new platform below the cave, and dig into it with the mole mitts. Dig straight up for a piece of heart, dig further north and boomerang the spark to create a fairy (catch it in your new empty bottle if you want) then continue digging east to a chest containing 50 mysterious shells. Go back out. Remember, although Biggoron is up there at the top of the falls, he can't give you anything until after you beat the game.
You can go now to South Hyrule Field.
South Hyrule Field
Talk to Tingle (you might have to talk to him a couple times) to get the Tingle Trophy. It replaces your Kinstones bag in the quest status screen but you can still look at the Kinstone pieces you have.
Note: if you dig to the left of the door of Link's house, a red rupee will appear. You can go inside and out again and the red rupee will come up each time you dig. Use this if you need to fill your wallet (mine was full at this point). You can go into the bottom left corner of Link's room and Ezlo will give you a hint about this.
Now go to Hyrule Town.
Unfortunately, you HAVE to play Anju's cucco catching game to get one of the game's pieces of heart. See earlier in the walkthrough for advice on how to win. Most of the following is a repeat from earlier. The stages get harder and harder, with the cuccos further and further away from Anju's cucco pen. Worse, more and more of the cuccos are golden, and they can fly out of your hands after a while. To get a gold cucco into the pen you need to keep throwing it ahead of you, rolling towards it and grabbing it, and repeat the process, otherwise it will fly out of your hands, probably in a direction away from the pen. If you want all the pieces of heart in the game, be patient and persistent.
A word of advice: for the harder rounds, first start the round and go around town scoping out the locations of the cuccos you'll need to catch to win. Once you've memorized their locations, go into a building and back out to quit that round. Then go back and start the round again and find the cuccos where you saw them earlier.
In the final round (3 cuccos in 55 seconds, two of them golden), once you get the two golden ones, you need to go west, swim across the stream, grab the white cucco, stand VERY close to the edge of the stream and throw the cucco across. If you're not careful, you'll throw it into the water and you'll have to start the round over again. Once you make it, you finally get that stupid piece of heart! After that you can still play but it's always the hardest round.
There's a piece of heart now accessible now that you've revived the Four Sword. Go into the Minish path in the school playground and split into four up above to push the large block up and out of the way, to reach the piece of heart and two chests containing red Kinstone pieces.
Now that you have the Roc's Cape, you can stand right under the town bell, jump and glide straight up, and you'll hit the bell and knock down a piece of heart.
Now that you have the Roc's Cape, there's a piece of heart you can reach in the cave where you got the power bracelets. The cats in the little garden area will be gone, so although it's a long way to reach the cave, you're not in danger of getting hurt. Once in the cave, get to the end of the short bridge, and jump across to the piece of heart. Then make your way back to the carpenters' cabin and grow to normal size again.
The last piece of heart I was missing (aside from the one for getting all the figurines) was one in the Minish woods. Warp there.
Shrink at the portal. Now that you have the flippers you can swim north from the portal, cross the bridge, continue north, and go into the leftmost tiny arch. (The middle and right ones lead to a blue Kinstone piece each.) Explore the cave from the leftmost arch to reach a chest containing a red Kinstone piece, and a piece of heart.
IMPORTANT: Before the final dungeon you should probably switch back to regular bombs.
This is it, it's time to return to Dark Hyrule Castle and beat Vaati. So, warp to Hyrule Town and head north to the castle garden, where you can just walk into Dark Hyrule Castle.