After you complete Unicorn's Cave, the Maku Tree says that he saw a vision of you going through vast ruins. He is referring to Tarm Ruins, which is locked and requires special treasures to open.
Get the Treasure Map
Go to the Subrosia Market. You can buy a silver card, called the Member's Card, for 5 ore chunks.
Now go to Horon Village and go into the shop. Go to the door behind the counter. The merchant will let you pass once he sees your Member's Card. Go through the door behind the counter and go downstairs. Buy a treasure map. You can also buy a bigger seed satchel and a Gasha Seed.
Now that you have the treasure map, when you press Select on the overworld to see the map, there are sparkles on it. There are hidden treasures at these locations that are required in order to enter Tarm Ruins.
Get the X-Shaped Jewel
Use the west exit of Horon Village and go south to the beach. Find the spot where there is a torch in the water. Light it with the slingshot and an Ember Seed. Cross the bridge that appears. Use a Mystery Seed on the question mark and it will turn into a Moldorm. Kill it and open the chest to get the X-Shaped Jewel.
Get the Round Jewel
Go to the entrance to Gnarled Root Dungeon (north of Impa's house) and go down the stairs south of the dungeon entrance. Swim west and south. Go up the stairs and into the tree. Talk to the old man to get the Round Jewel.
Get the Square Jewel
In a Normal Game
Go to Spool Swamp, to the screen south of the entrance to Poison Moth's Lair. Use the tree stump there to turn it into winter. Go to the southeastern part of Spool Swamp where there is a dead tree that you can walk past and dig some snow piles to reach a cracked wall. Bomb it and go in to get the Square Jewel.
In a Linked Game
Go to the tree stump where Din danced with you (east of the entrance to Gnarled Root Dungeon). Use the stump to turn it into summer, then go south and west. Bomb the rocks and go in to get the Pyramid Jewel.
Get the Pyramid Jewel
In a Normal Game
Go to Mt. Cucco by the secret passage next to Ingo's house in Sunken City. Use a tree stump to turn it into spring. Go to the screen between the two spitting business scrubs and use the flower to go to the upper level. Go east. In the screen with Dancing Dragon Dungeon, dive into the water in the upper-right corner. Go through the underwater tunnel to get the Pyramid Jewel.
In a Linked Game
Go to the Woods of Winter and find the house of the girl who gave you the shovel. This is Holly's house. Go west from Holly's house and into the cave. Swim to the chest and open it. Vire pops out and makes the pyramid jewel appear on the ledge. Go to Sunken City and swim to the southern part. Dive into the square of deep water bordered by shallow water. You end up back in the cave with the Pyramid Jewel. Go to the right and walk along the ledge to get the jewel.
Put the Jewels in Place
Now you must go to Tarm Ruins and put the four jewels into the slots. Warp to Spool Swamp and go east. Walk north through the log. Push on each of the empty jewel slots to put your jewels into place. The gate to Tarm Ruins will open. Go through.
Reach the Lost Woods
Get on the stump and turn it into summer. Climb the vines and kill the Darknuts. Drop down the ledge and go west.
Push the Armos statue to make stairs appear.
Go up the vines and go east, then go down the vines. Use the tree stump to turn it into winter.
Go west and go up the stairs, then walk on the snow to reach the stairs to exit north.
Push the bottom two Armos statues to the right. Use the tree stump to turn it to Autumn. Walk on the Armos statues and go east.
Pick the mushrooms and go down the stairs. Use your shield to bounce two nuts back at the Deku Scrub. Talk to it to learn the path through the woods: west, south, east, north, taking each step in a warmer season.
Pick the mushrooms and go west. Walk on the Armos statues and exit north.
Use the tree stump to turn it to Autumn. Pick the mushrooms and go west.
Kill the Like Likes. The cracked wall leads to a fairy fountain. Use it if you need to replenish your health. Go south, then west. You are now in the Lost Woods.
Get out of the Lost Woods
Get on the stump and turn it to Winter, then go west. Get on the stump and turn it to Autumn, then go south. Get on the stump and turn it to spring, then go east. Get on the stump and turn it to summer, then go north.
Reach the Ancient Ruins
Go north past the unmoving statues. Go north up the stairs. There is a Gale Seed tree here, so you can warp back here any time. Go east and turn it to Autumn, then go west and pick the mushrooms, then burn the tree with an Ember Seed. Go down the stairs and open the chest to get a Gasha seed. Go back outside.
Go east. Use the stump to turn it to winter. Go south, east, and north, then walk up the snow ramp to exit north.
Go west, then push the bottommost Armos statue to the side. Jump down the ledge and turn it to spring. Walk onto the flower to go back up to the ledge. Cut the flower and cross the bridge. A mysterious hooded individual will warn you that rescuing Din will cause bad things to happen.
Go into the building to enter the next dungeon: Ancient Ruins.