Instructions
Defeat the Dark Wizard Agahnim for the first time. You will meet this foe for the first time atop Hyrule Tower once you have obtained the Master Sword. This fight is part of the normal progression of the game. Once you have defeated him, he will draw you into the Dark World. Hyrule Tower will become a step pyramid.
Climb down the pyramid and head east.
Travel northeast in the direction of Zora's Domain. Eventually, you will come across a long waterfall with a ring of stones nearby.
Throw anything -- such as a bomb, a sign or a skull -- into the ring of stones. A fish will pop up out of the water and complain that you've disturbed his sleep. He will toss you the Quake Medallion as a bribe to leave him alone.