Since Garrett can't see, he uses sounds to explore the game's world and to complete its challenges. He puts two speakers right in front of him, one on his left and one on his right, which help him know what's happening around him in the virtual world. It aslo lets him use in-game items like Link's Sword, to help him move around the world. For instance, he'll hit it against a wall to lead him along passageways.
Garett used save states that let him trial-and-error through the game's levels. He used them to complete the infamously difficult Water Temple.
Garrett posted the final video of his playthrough on January 2, saying, "It has been long in the making, but I have finally accomplished my goal."
Great job, Garett. This is an amazing achievement. (Applause).