The Temple is a pyramid surrounded by trees. Before you can gain access you need to find the Keystone. To do this first go to the house near the coastline to the East of Gongaga, the man there should tell you that he has sold it to Dio, the owner of the Gold Saucer.
Next head to the Gold Saucer and enter the battle square. Look in Dio's Hall of Fame and you should find the Keystone, but before you can take it you must fight in a battle. It doesn't matter if you lose because Dio will give you the Keystone anyway. Once you have it head back to the Ancient Temple.
Once you have found the Keystone you can enter the main Temple. This is a confusing maze of paths and walkways. When you first enter you should spot a purple colored person, you need to follow him. When you do eventually catch up with him you can buy Items of him, restore your parties HP and MP, or save your game.
You will see quite a few boxes and items lying around, try to pick up all of them if you can.
You then need to make your way towards an exit which is located near the bottom of the screen, when you enter it you will come up against some rolling rocks. This seems impossible at first glance, but after a little practise you should be able to do it all in one go.
Try to work out where the gaps in the rocks will fall, and stand in one place. If the rocks hit you, you will be returned to the start. Once you reach the end the rock will cease to roll, and then Sephiroth will turn up. Once he has gone, use the Purple coloured Ancient to restore your health and save your game.
The next place you will come to is a Clock Face, known as the Cetra Shrine. This is a puzzle that requires you to spin the hands into a position that will let you move from entrance to entrance. The following is the list of what you will find in each doorway :-
When you enter doorway VI you have to chase the Purple Ancient about, once you catch him you will have the key to unlock the doorway, and you can also use him to restore your HP and MP, and save your game.
When you enter the locked door you will meet Sephiroth again and you will learn more about the story. At some point during this you will come up against a Boss (see below).
When you have defeated the boss you should go back out of the door, and head into dooway XII. I would advise that you restore your health and save your game before entering XII, because you are about to come up against the toughest Boss so far in the game.
Once this boss is defeated Sephiroth will turn up again demanding the Black Materia you have just taken from the Temple, there is nothing you can do that will stop what happens here. After everything is over, you will find yourself in Gongaga, and you should now head towards the Bone Village.
When you enter the locked door in doorway VI you will have to fight Red Dragon. When he dies he leaves behind the Bahamut Summon Materia.
When you try to leave through doorway XII you will have to fight Demon Gate.