After you defeat Moragia, the creature that covered Death Mountain Crater, you jump in and find yourself in the Eldin Canyon Depths.
Follow the Voice
After you jump into Death Mountain Chasm, look for the Lightroot and land there. This is Mustis Lightroot. Examine the glowing object there to light up the area. Then talk to Yunobo, who is to the southwest.
After a cutscene, go west to where the voice said to go. Notice that there are statues here. You can follow them on your way to the destination. Have Yunobo break the rocks in the way. To the west, you find some Zonai devices, including a partially assembled vehicle with big wheels. Near that, there is a glowing rock. Have Yunobo destroy the rock to find another partially assembled vehicle, this time with a Steering Stick as well. Attach one of the nearby wheels, and attach a light to the front, then check on the controls to start the vehicle.
Go north to where you can see a Device Dispenser. From there, have Yunobo destroy the rock that the statue is facing. Keep following the statues and you will go past a mining camp with Bokoblins and a Moblin. Continue going south until you find a slope that goes west. There will be more rocks in the way, so have Yunobo break them.
In the area beyond where the rocks were, go north to where you can see a Lightroot. You will have to battle some Bokoblins and a Boss Bokoblin, but Yunobo will help. Then go north and activate Misisi Lightroot.
If you still have the vehicle, you can drive west from here over the lava (it's shallow) to reach the bridge to the quest destination. Otherwise, go west from Misisi Lightroot to find a hot spring, then go southwest and you will find a covered area with some Horriblins. You can ignore them or battle them, then go west, then north when you can, to where you can see a ramp leading up to a larger ramp. At the top, go west to the quest destination. Talk to Yunobo and there will be a cutscene. Then have Yunobo break the glowing rock at the entrance. Go through to enter the Fire Temple.