There is a video on YouTube called «Ocarina of Time 3D Major Glitch #2: How to enter Ganondorf's Castle as Young Link» which explains how to do this glitch. (at the beginning of the video, Link is in the Great Fairy Fountain next to Zora's domain and you will need Farore's Wind, Bombs, and you must have finished the Dodongo's Cavern Dungeon. You also need to know the «Any Items anywhere» glitch.)