After infusing the Picori Blade with two elements, you encounter Vaati and must find a way to the next element.
North Hyrule Field
Here you will be confronted by Vaati. He and Ezlo will have words with each other and you learn a little something about the plot. Vaati traps you in with a couple moblins. Beat them and you'll be free, and Vaati will be gone. Listen to Ezlo and you will learn more about him, and about Vaati, and the light force.
Now for a detour to get a new item. Head for Lon Lon Ranch to the east.
Lon Lon Ranch
Right as you enter you'll see a hole above you. Use the Cane of Pacci in it and get in to be boosted to the ledge above. Continue up to Veil Falls.
Head all the way right to find a Piece of Heart. Then before going back up the stairs, there's an exit south you can take, and if you fused Kinstones a chest might be here containing 200 rupees. Then exit to get back to Lon Lon Ranch.
Lon Lon Ranch
Head all the way south to the Eastern Hills.
Eastern HillsHead down the stairs, and to your right will be a hole you can use to get up onto the ledge. Do it, and exit right into another area of the Minish Woods.
Enter the tree. Inside, agree to give the fairy all of your rupees. In return, she gives you a big wallet that can carry 500 rupees (and you don't lose any of the rupees you were carrying).
Now that you have a bigger wallet, you can look for one of the chests you revealed with a fusion that contains 200 rupees. Use the checklist to find the locations of these chests. You can also defeat the golden rope in the Eastern Woods if it's there to win 200 rupees.
Before moving on, if you have any Kinstone fusions left to do, go back to the Eastern Hills and use an exit here to reach the Minish Woods (it's best if you take the southernmost exit, you reach the Minish Village more quickly).
Do any Kinstone fusions you can with the Minish in the woods and the village. If you did all the fusions in town and on Mt. Crenel you should be done with the first 22 fusions that you can do before heading for Castor Wilds.
Get to South Hyrule Field.
South Hyrule Field
Use the warp tile you created north of Link's house to reach another house. The two chests in the first room contain a green Kinstone piece each. In the upper room, the chest contains another green Kinstone piece. Now, it's IMPORTANT that you use the gust jar to suck the ghost away from Gregal. You have to move side to side while using the jar to keep moving with the ghost. Do it from below, you won't get it to work from the side. Once you do it long enough the ghost will be sucked away. If you don't do this before a certain point in the game, you will miss out on a very helpful item later. Once it's gone, talk to Gregal for 100 mysterious shells. Now leave.
You want to make your way to Mt. Crenel.
There's a piece of heart and sword technique and some treasures to be had here, now that you've infused your sword with the two elements. From the entrance go north to the climbing wall and go up. Keep climbing up the rock walls until you reach a cave. Inside are glowing tiles you can use to split in two. Split on the two top corners of the tiles and go through the gaps to step on the two switches. Inside, grab the piece of heart. The two chests contain a total of 100 rupees. Train with Grayblade to learn the roll attack.
Again, if you're missing any fusions, you may have to return to Melari's Mines.
When you're ready, go west from Hyrule Town to Trilby Highlands.
Head north and up the bridge you see, then exit the screen right to reach a new area of North Hyrule Field.
North Hyrule Field
Beat the monster, then use the Cane of Pacci in the hole to get up on the ledge. Flip the switch to connect the bridge. Now return to Trilby Highlands.
Head south until you reach a ladder. Go down. Use the White Sword to split in two such that the two Links can push the block left and out of the way. Go up the ladder at the other end.
Get rid of the monsters around, then bomb the wall above for a red Kinstone piece. Go south now, ignore the tree hut, then east. First go north. There are two rocks next to each other to the right of the steps, put a bomb between the two to blow up the wall. Inside are some fairies, you can use an empty bottle to catch one (they revive you if you die). Go back out and up the stairs, and push the rock into the hole so you can get down to this area more easily next time.
Now go south and exit south to the Western Wood. A cutscene will take place in which you see Vaati approach the king in Hyrule Castle, and later the king is acting fishy, demaning his soldiers search for the light force or face imprisonment. Gee, wonder what that means.
After this point, new fusions open up.
Anyway, back in the Western Wood, go south, push the rock into the hole to make a shortcut to South Hyrule Field (where Link's house is). Next, head west, then north to Castor Wilds.