Firstly, after beating the Forest Temple, go to the Goron City as an adult, there you will see a younger rolling goron. Use your bomb to stop him from rolling, then you ask him some questions leading up to the doors on the bottom floor opening.
After getting the Fire Tunic, wear it. Then go to the Darunias Room, there you'll find a statue in the middle, pull it back to reveal the hole to the Death Mountain Crater.
Make sure you have the Hookshot, as you will need it to cross the broken bridge to the right of the entrance, once there you will learn the "Bolero Of Fire" from Sheik, thus allowing you to teleport to the crater at any given time.
Once you acquire the song, make sure you save it, common mistake.
Then make your way up towards the entrance where you will find a hole, you can either jump risking some loss of health, or you can simply climb the ladder down.
Then you just walk upwards and voila!!! you are at the Fire Temple.
If you don't have epona freed from the ranch just go into the ranch talk to Mr. Ingo and he will charge you to ride a horse, right after that starts use eponas song and epona will come to you. jump over the fences right in the middle, after doing this several times then you can challenge Mr. Ingo to a race. If you beat him at both races that he challenges you to then you get to keep the horse. then simply jump right over the gate that the guy closes or right over the walls around the horses area.