You have defeated the Goron chief in a sumo wrestling match. You must now find the Goron mines and defeat Fyrus. You should have a heat resistant shield, and it is a good idea to stock up on health potions and other items as needed in the nearby shops. (The heat resistant shield is purchased in Karaiko Village.)
Go southeast of the village until you come to the entrance of the cavern. Enter the mines and cross a lava pit wherer bursts of fire come up from the floor. Journey carefully as you travel across the floor between the bursts. Get to the other side and break the barrier on the wall.
Flip the switch and run across the room where there is another switch on the wall. Step on the switch and move back quickly, unless you prefer to have your pointy ears singed. Remove the boots from your inventory. Move across the grate on the other side of the lava pits. The iron surface gives way to a stone surface. Step on the reddish platform to reveal a small chest.
Take the key from the small chest in this room. Walk up the ramp that leads up to the rotating platform. Wait for the platform to stop and jump across to the other side. You should come to a locked reddish-brown wooden door. Open the door using the key. Continue following this path remembering to time your progress appropriately and use the platforms to jump across the lava. When you find a chain, pull on it as far as it will go to reveal another door. Defeat the lizard in this area by hitting its tail. If you take too long to pull the chain to its full length, the door in the wall closes and you must go back to the chain to open it.
The next room is filled with water. Put the iron boots back on and sink to the floor. Press the switch nearby and a magnet will pull you to the ceiling. You can use the iron boots to walk across the ceiling.
An older Goron awaits you in the next room. He will give you the first key shard when you speak to him. You will need three key shards to open the final door of this temple. Get the map from the chest behind the NPC in this room. Use the ladder to get to the next area.
Break the pots on the floor again until you find the spirit Ocococo. (This handy little spirit helps you exit dungeons quickly, by the way.) Equip your iron boots before entering the next room. Find the switch in this room that activates the magnet. Walk across the ceiling until you come to the middle of the two lit torches and drop down. The lit torches open the net door.