Instructions
Exit the final Temple, the Spirit Temple. Equip the Ocarina from the "Start" menu and assign it to one of the "C" buttons.
Play the "Song of Time" (press "Start" and move sideways until you get to the song menu screen to be reminded how to play it on your Ocarina) to warp to the Temple of Time. Watch the cut-scene. You will be given the "Light Arrows" and you will be prompted to enter Ganon's Castle.
Exit the Temple of Time and return to the dilapidated Market Square. Go north and follow the path to reach Ganon's Castle, which has been inaccessible until this point.
Approach the castle; here Rauru, one of the sages, will appear and inform you that he and the other sages will create a bridge for you to gain entrance to the castle so you can defeat Ganon.
Walk across the newly created "Bridge of Light" to enter the castle.