To be able to unlock trading with Pokemon Ruby, Sapphire and Emerald, you need to have accomplished the following things
- Beaten the Elite 4
- Retrieved the Ruby and the Sapphire
- Have national pokedex (collect 60 pokemon, then speak to oak)
Please read the whole procedure below:
When you beat Cinnabar Island's Gym leader Blaine, you will be approached by Bill, who will ask you if you want to go with him to the Sevii Islands. I dunno if most people know this, but you can refuse to do this now. Bill will continue to stay waiting for you in the Cinnabar Island's Pokemon Center. Whether you want to do this now, or after you beat the Elite 4 the results will be the same.
Bill will take you to meet Celio who is attempting to initiate connection between the Sevii Island's computers and the Kanto region (the place you came from). While the guys chit-chat and work together, you will be asked to take a METEORITE to the Game Corner's Owner on Island Two. Bill will give you his Tri-pass so that you can board the ferry by the dock outside.
When you arrive on Two Island's Game Corner, the Owner will tell you that his daughter Lostelle is lost. You must go to Three Island to find her. Go back to the ferry to go to Three Island. Go north then go west, you need to pass a series of grassy areas, pass the Bond Bridge, and pass the beach to reach the Berry Forest.
You need to surf toward the left in the forest to find the place where Lostelle is, you need to battle a Level 30 Hypno. When you do, Lostelle and dad will be delighted, and you can now give the METEORITE to her father. He will give you a Moonstone in return.
Now your job here is done for now. Next thing to do is go back to One Island and Bill will bring you back to Cinnabar. He will also tell you, you can go back to the Sevii Islands anytime you need to by going to Vermilion City's port.
Next time you go to Island One, Celio will tell you he needs the gemstones to link to faraway regions. He means the Hoenn region into Lanette's computer. You need to get out of the Poke Center and walk towards the EAST and surf to the NORTH. Keep going on Kindle Road battling all trainers and over-coming obstacles.
You will need a Pokemon that knows STRENGTH and ROCK SMASH. To get HM Rock Smash go to the hot springs (its a cave north of Kindle Road where you heal your Pokemon), there is a man there who will give you HM Rock Smash. Get out and go NORTH once more, surf and you will arrive in Mount Ember. A bit to the right you will see that Team Rocket has punched a hole through the wall of the cave. In their conversation you manage to capture the first PASSWORD to their hideout: "GOLDEEN need log."
Battle both Rockets and the cave is yours to explore! You will find Ruby inside.
Take it to Celio and he will tell you he needs another gemstone to get his machine to work. Go to Four Island. Get into the Poke Center and Gary will be there to brag once more about how if he is not able to complete his PokeDex, that there is no way you can complete yours. So untrue, but anyway. Pick up a pokemon that can learn waterfall while your here. It will save you a trip.
Get out of the Poke Center and go toward the back of Lorelei's home, and enter the Icefall Cave. Use surf and navigate to the left cave entrance. Cross over the first ice patch then take the left path on the next patch. Walk on the topmost icepatch twice to fall down. Go up the ladder and move down to the second ice patch. Break through the patch. Move up, right, down, left, down to get to the ladder. Climb the ladder and pick up the nearest item on the ground. This is HM Waterfall. Give it to a Pokemon and go up the Waterfall, go down the stairs then up into the cave. You will find Lorelei battling Team Rocket there, blah blah, fight win next scene.
Lorelei will demand the Rockets to tell her where the Pokemon they've caught are. He will say that the Pokemon are in the Rocket Warehouse on Five Island. He also says is near impossible to enter it as you need TWO passwords. You already know one, you need to go and find the other. So if you head to Island Five you will be wasting your time. The second password can be found on SIX ISLAND.
Go to Six Island and walk EAST of the Pokemon Center and go through a long maze to find what is known as the Dotted Hole. Use the move CUT on the door to get in. Go down the hole and you will find yourself in a 4 hole puzzle!
To solve it, go through the holes in this order:
The Sapphire is there, yay finally you can get it and... oh wait its game freak, someone keeps you from getting it!
The nerd who snatches your Sapphire will tell you the password to the Team Rocket hideout on Island Five. Which is "Yes, nah, CHANSEY."
Finally, you must go to Five Island to retrieve the Sapphire in the Team Rocket's hideout. To get there you must stay on the sandy area once you get out of the ferry boat and go toward the right. Keep going and battle 3 rockets.
When you get to the door you will be able to enter with your two passwords. To be able to get the Sapphire you need to go through the conveyor belt puzzle and battle everyone. Take the topmost arrow path to your left first, go left again, beat the nice lady and continue to the next patch of arrows, take the left patch. Go north, stand to the left of the good fellow then take the arrow patch to the left. Avoid touching the arrows and weave your way northward. once you hit a wall (actual wall), go all the way to your right, and down the nearest arrow patch. Go down, greet the wonderful gentlemen and keep going. Greet the endearing woman to the right go north and face the admin then go north. Finally after long last you can... beat up the nerd... FINALLY AFTER LONG LAST get your Sapphire!
Take it to Island One and give it to Celio. He will be so ecstatic about it, and will tell you he has initiated communication with Lannette's computer! Now you will be able to trade Pokemon with Pokemon Ruby, Sapphire and Emerald.
Congratulations, after this you can consider that you have successfully passed the Pokemon Fire Red and Leaf Green games.