First, you MUST have the Coral Earring, which you get through the games main storyline. Once you have that, proceed to Hena's Fishing Hole in Upper Zora's River, near Iza's Rapid Ride. Talk to Hena, and go fishing. Once you have caught a Hyrule Bass, a Hylian Pike and an Ordon Catfish, go to the North-West corner of The Fishing Hole, beyond the place where you cross a bridge to catch a bottle (See Below). Fish with a Bobber, (The rod you keep) and keep trying. Eventually, you will catch the fabeled Sinking Lure! The Sinking Lure can be used to catch fish incredibly easily, and even the fabled Hylian Loach! But be careful, if Hena sees it, she will confiscate it, but you can get it back by fishing with the rod again in the same place.