Pokémon FireRed and LeafGreen/Four Island

One Island
In order to truly finish the game, you'll need to explore all of the Sevii Islands. To begin, go talk to Celio, Bill's friend on One Island. To get there, take the Ferry from Vermilion City. If you forgot, Celio's in the Pokémon Center. Celio asks you to help him find a gem that will make his Pokémon trading machine work. Go to Mt. Ember to find the gem.

In search of the gem
Go to Mt. Ember, and go right after going up the stairs. You'll find two Rocket members talking to each other about the secret passwords ("Goldeen need log"), blocking an entrance. Go up to them and battle both of them. They'll leave after you defeat them. Go inside the cave and follow the path. You'll go through a boulder puzzle, and then reach a fork in the road. Take the south road; the other road takes you out. The next room is full of Braille messages. Go on to the next room.

Pick up the Ruby in the center of the room. Leave Mt. Ember, and go back to Celio. Hand him the Ruby, and he'll want you to go on another mission. You'll hand him your Tri-Pass, and he'll give you the Rainbow Pass, used for exploring the rest of the islands. Now take the Ferry to Four Island.

Four Island
Once you arrive at Four Island, also known as Floe Island, Gary will first come to talk to you. First, go north a bit and break the rock. Keep going north for a Star Piece. Return to where you were and go to the house next to the Poké Mart. Talk to the person inside, and he'll apply a Hall of Fame Sticker to your Trainer's Card. In the upper-right hand corner of the island is a house with a Move Tutor inside. He teaches. Next to that house is the Day-Care Center. Leave your Pokémon here to let it breed or raise its levels. Next, you surf across the pond for a Stardust, and then surf to the other side to enter the Icefall Cave.

Icefall Cave
Once you're in Icefall Cave, surf across the water and go deeper. In this room, you'll find a puzzle with many blue ice tiles. They are just thin pieces of ice that crack the first time you step on them, and break the second time. This is a fairly easy puzzle. You want to get to the bottom-right corner of this room, where you'll find HM07 - Waterfall. You want to fall through certain holes to get to the HM, and after some trial and error, you'll get to it.

After getting the HM, teach it to a Pokémon, exit this room and go up the waterfall. Go up the ladder, into the entrance, and you'll find Lorelei defending the Pokémon of this cave from Team Rocket. She'll then drag you into the argument, and you'll have to fight a Rocket member. After the battle, the Rocket member tells you that they've kept the stolen Pokémon in the Rocket Warehouse on Five Island. The Rocket Warehouse requires two passwords, and you only have one from eavesdropping on two previous Rockets. Instead of going to Five Island now, you'll need to first go to Six Island to find out the other password.