After you follow the Subrosian girl in the Eastern Suburbs, you reach Subrosia by a hidden portal.
Get the Boomerang
In the first screen of Subrosia, if you talk to the Subrosians, you learn that the temple fell into the East Plaza. Don't go there yet. You should get the boomerang first.
In the first screen of Subrosia, go down the stairs on the left. Follow the path past the Subrosian hot springs and go south to some stairs. You reach the Subrosia Dance Hall. Talk to the red Subrosian and agree to dance. You have to repeat after the red Subrosian. They will do a set of three moves, then you have to repeat them, then they'll do another set of three, and so on. If you dance well, they give you the boomerang.
Get the Rod of Seasons
Go east until you reach the temple. In the entrance, the Season Spirits will speak to you, asking you to get the Rod of Seasons and then visit them in the four corner towers of this temple.
Go straight north and walk onto the altar to receive the Rod of Seasons. You can use it to change the seasons!
Get the Power of Winter
Go to the southeast tower of the temple. Inside, use the boomerang to hit the crystal switch (if you don't have the boomerang, read the how to get the boomerang). Follow the path through the tower until you reach a statue. Go up to it.
The Spirit of Winter will add the power of Winter to the Rod of Seasons. You can change the seasons by standing on a tree stump and swinging the Rod of Seasons.
Outside, the Maku Tree will speak to you. There is nothing else to do here in Subrosia for now unless you want to win prizes in the Dance Hall or cut grass to collect ore chunks. Otherwise, go back to the portal to return to Holodrum.
Get the Shovel
When you go east, there will be a cutscene. Afterward, stand on the tree stump and swing the Rod of Seasons. It will become winter, changing the landscape. The water that was blocking your path to the east is now ice, so go east to the next screen. Go east again, then south, then east, and you will see a Piece of Heart that you can't reach right now. Go north from there, and keep going north until you see a house. Go west until you see a broken wall. Bomb it and go inside to find a treasure chest in the northeast corner that contains 30 rupees.
Go back outside, and go east to where there is a tree stump. Stand on it and use the Rod of Seasons, then go up the stairs in the same screen as the tree stump. Go east and walk along the snow, then jump into the chimney of the house. Talk to the girl and you will get the Shovel. Exit her house and use the shovel to get rid of the snow.
Get Mystery Seeds
Now that you have the shovel, go south until you can't go farther south, then go west. If your path is blocked, stand on the tree stump and use the Rod of Seasons. Then go west, then north up the path on the right, and you find your path blocked by piles of snow. Dig the snow piles and go east.
Here you find a Mystery Seed tree. Hit the tree with your sword to collect the seeds. You can use the seeds on owl statues for advice. The statue next to the tree just tells you to feed it Mystery Seeds.
Go to the Next Dungeon
Now go north, then go west to find the entrance to Snake's Remains.