If you followed this walkthrough's dungeons completion order, you now own two pieces of Master Ore. So go back to Hyrule, to the blacksmith and talk to him. He notices your two pieces of ore and upgrades your sword to level 2! Go back to Kakariko Village, go north, behind Sahasrahla's house and enter the fissure.
When arriving at Lorule, Hilda calls you. She says that the Skull Forest lies in front of you and warns you against its dangers. Turn into a painting, go right and find a maiamai. Merge again, go further right and exit near the Piece of Heart waiting for you! Go back behind Sahasrahla's house, then continue north, then northeast to bypass the forest. Go in the "V" formed by some trees and cut the bushes hiding a maiamai. Continue north again, and if you have the Pegasus Boots, rush to the tree to the south of the ruined house to make another maiamai fall. Notice the heart on the roof of this house, then head southeast, climb the ladder, pick up the cucco on the path, continue to the far north, then throw yourself towards southwest and fly to the flat roof where the Piece of Heart lies. Enter the forest through the east or southeast entrance, then follow the path to a weather vane and touch it to activate it. Enter the skull on the left.