Go to the screen where you first met Din's troupe and go down. Go across the gap and into the water and go down again. Go right and onto the island, avoiding the fish, and into the portal. Head south until you hitn the bottom of the map and go left twice, up, and right to the Bomb Flower. Use the Power Bracelet to pick it up.

Now, go to the Temple of Seasons and go to the top right tower, jumping across the lava river, and talk to the Subrosian with a headband. He'll take the Bomb Flower and bomb the boulders. Enter the tower and recieve the Power of Autumn.

Go back to the portal you first came in from. Head up and change the Season to Fall on the stump and go up and right to find Unicorn's Cave. Lift the mushrooms and throw them out of your way to get to the Dungeon. Enter it.