Head southeast from the Gold Saucer to a small island where the temple is located. When you go inside, you find Tseng who has been badly wounded. He turns over the Keystone. Examine the alter to continue on, but be sure you're prepared because the only way out is to go all the way through the temple.
You'll start at the beginning of a Escher-esque maze. Follow the little bearded guy around and pick up the Trident in a chest and a Mind Source on the way. He leads you to a small room where there's a Silver Rifle in a chest. The bearded guy turns out to be an Ancient and will sell you items, restore your HP and MP, and save your game.
Go back out and climb down the vines, but this time head forward through the archway to pick up the Turbo Ether sitting there. Continuing on, the next chest contains a Rocket Punch. You'll see another Ancient run into another doorway, but don't follow him just yet. Go through the arch and down, then climb up the vines to reach a Luck Plus. Now go in the doorway where the Ancient went.
Inside there are some rolling 'U' shaped boulders. With some timing, you can place yourself where the opening of the 'U' will be so you can advance without getting squashed. Pick up the Morph at the purple pool and keep going to the end until the boulders stop. Follow Aeris back to the pool to see a scene about the Turks and Sephiroth. Another Ancient will be waiting at the end of the path for you to rest and save.
The next room is a giant clock. Move the hands around to create a path to the various rooms:
If you get knocked off the clock by the second hand, you'll have to fight 2 Ancient Dragons but there's also a chest with the Nail Bat in it. The door in Room XII won't open yet, so go to Room VI when you're done getting the items from the other rooms.
In this room, you must catch the Ancient that holds the key to the door by entering the doorway that he will come out of. The first door he comes out of is the 2nd door from the right on the middle level. At the bottom of the room is a chest with the Work Glove in it. Once you catch the Ancient, he'll unlock the door and let you rest and save.
The next room is the corridor you saw in the flashback with the Turks and Sephiroth. At the end, Sephiroth explains his plan and you'll have to fight a Red Dragon. Pick up the Bahamut on the floor and then examine the alter at the end of the corridor again. Cait Sith volunteers to get the black materia, so head out to the exit in Room XII. Before you can get out you'll have to fight the Demons Gate.
After you exit, Cait Sith turns the temple into the Black Materia. But just as Cloud picks it up, Sephiroth shows up and forces him to hand it over. In a vision, you see Aeris in the Sleeping Forest with Sephiroth after her. When you wake up, you'll be in Gongaga.