After obtaining two of the elements, you should return to Hyrule Castle to infuse the Picori Blade.
Shrink down and enter Melari's Mines. If you haven't completed the first 22 fusions, there may well be someone (or more) in the Mines willing to fuse with you. Check everyone to find out.
Go to Melari where you found him before and he'll give you the repaired Picori Blade (White Sword). Your goal now is to find the Elemental Sanctuary in Hyrule Castle.
If you're still short one or two fusions, you may want to go right back into the Mines and check around again to see if anyone wants to fuse now.
To get off the mountain, grow to normal size then drop down the ledge below the shrinking portal. You may want to take the left-hand ladder down and go west to the climbing wall to visit the Crenel Hermit before you go, he may have a Kinstone piece or two to fuse if you didn't fuse with him already. Otherwise go right and keep heading down, the path out of here is fairly straightforward. Make your way to Hyrule Town.
Do a lot of Kinstone fusions here if you didn't already. You'll want to make sure you get them out of the way before you head for Castor Wilds. If you can't fuse with anyone in town, that probably means you've done all the ones in Hyrule Town. Hopefully you fused with as many people on Mt. Crenel as you could, otherwise you may have to go back there or go to the Minish Woods to complete the 22.
Go to Swiftblade's wooden cabin to learn a new technique.
Buy the boomerang in the shop if you didn't already.
In Mama's Cafe and Romio's house are giant pots, use the Cane of Pacci to flip it over and it turns into a shrinking portal! You can't get out the human-sized door but you can get out through the small blue arch near the corner. Look for tiny ladders and holes everywhere to discover areas you couldn't reach at normal size. At Minish size, you can talk to animals, but most (if not all) cats will try to attack you. Some animals may be willing to fuse Kinstones with you when you're small.
There is a chick atop Swiftblade's dojo that you can talk to. To the right of Romio and Julietta's house is a dog you can also talk to. Both Romio and Julietta have pets. There's also a dog in the west end of town you have to talk to to proceed, more on that later.
There is a vine north of Romio and Julietta's houses that leads up to the higher level where the inn is. Around the back of the inn is a path marked with flowers. If you go down it, you enter a room with a shrinking portal and a piece of heart. Grow and grab the heart piece, then shrink again and make your way back to Romio's house to grow again.
Also while small, you can go into the back of Wheaton and Pita's bakery, and if you climb up and past the puffs of steam to reach a hole, there's a chest in the rafters area containing 100 rupees.
There is a wooden plank in the southwest area of town that you can now cross at Minish size (shrink at the cafe this time). Go across and go up to the dog that is blocking the stairway. Talk to it and it will move. This means you can access this area at normal size.
To get back to normal size now, There's a little path of flowers leading into the top of the cabin with the waterwheel that will lead you inside. There is a Minish house you can reach by climbing the tiny ladder, but the Minish doesn't say anything important (but might have a Kinstone to fuse). In the cabin is a shrinking portal, so use it to grow, and go outside.
At the bottom of this area is Carlov's hut, where you can bet mysterious shells for figurines. See the "Carlov's Figurines" section for more details.
North Hyrule Field
When you're ready, head north to North Hyrule Field. To the northwest is a bombable rock. Get it out of the way and climb down the ladder. Go down the stairs for a piece of heart. Leave and continue on to Hyrule Castle.
Head straight north through the castle, then in the above room go around one side and past a set of stairs to the throne room and talk to the king and Potho. Leave the throne room the way you came in, then take one of the staircases down that are to the left and right of the throne room entrance. Go around south, and you'll see a door in the wall above you. Go through the door and continue north to reach the Elemental Sanctuary. Go up the stairs to the raised area, stand above the sword pedestal, and press R to drop the sword in. It will be infused with your two elements. A tablet will appear above, and Ezlo will read it, explaining your new power.
Now, you have the special power of being able to split in two when you stand on special glowing tiles. Try it on the two glowing tiles below the tablet. Stand on one, power up your sword and wait for the green meter to max out, then keep your sword powered up and walk to the other tile. You'll split. Your ghost image will move around as you do, but will always remain the same space apart from you. If it hits a wall or barrier, it will disappear. It will also disappear when the green meter runs out.
There are four glowing tiles right where you entered, use them in such a way that your double is spaced apart from you so the two of you can step on the two switches at the same time. The door will open.
Now you can leave the castle and begin your search for the next element.