As you know, it is possible for Ruby/Sapphire to trade with Leaf Green and Fire Red. However, for these games to be able to trade to each other, you require 2 stones that must be given to Celio so that he can work his long-distance trade link machine.

To obtain the Ruby, you must defeat the Elite Four and have 60 Pok'emon owned on your Pok'edex. Professor Oak will give you a National Pok'edex. Now travel to 1 Island and talk to Celio.

After talking to him, go to Mt. Ember and battle the 2 Rockets who are guarding a cave. Once they are defeated, enter the cave and make your way to a room with a gem stone in the middle of it. Grab it and give it to Celio.

He will give you a RAINBOW PASS. Use this to get to 4 Island. Enter the Ice Cave and go up the Waterfall. Enter the door and make your way into a new room where Lorelei will be standing. When a Rocket challenges you, defeat him and then make your way to 6 Island. Go South to the Dotted Hole and Cut down the door. Go inside and jump down the hole in the middle of the room.

Now you will be in a weird room with 4 holes and braille in the middle. Jump down the holes in this order: UP, LEFT, RIGHT, DOWN. You will see the Sapphire, when you try to grab it, a Scientist will snatch it. He will tell you the second password to the Rockets Warehouse.

Go to 5 Island and go to the Meadows. Travel South and find the Rockets Warehouse. Once found, enter and make you way to the Scientist. It may be hard to get to him with the confusing magic tiles, but you will get there.

Once you reach him, battle him and once he is defeated he will give you the Sapphire. Now return to 1 Island and give it to Celio.

Now he will establish a link with the Hoenn Region and trading between Ruby/Sapphire and Leaf Green/Fire Red will be enabled!