Desert Colossus:
Travel to the Desert Colossus. You'll need to cross the Haunted Wasteland using the Longshot and the Lens of Truth. Play Requiem of Spirit (learned from Sheik) to warp to the Spirit Temple entrance.
Spirit Temple (Child & Adult):
This dungeon requires you to switch between Child Link and Adult Link. As Child Link, obtain the Silver Gauntlets. As Adult Link, use the Silver Gauntlets to push large blocks. The boss is Twinrova. Use the Mirror Shield to absorb and reflect their elemental attacks. After defeating them, Nabooru awakens as the Sage of Spirit, granting you the Spirit Medallion.