Things You'll Need
Instructions
Using "Pokemon Emerald"
Complete the Hoenn Pokedex by capturing all 200 species of Pokemon available in "Pokemon Emerald." You do not have to capture the rare Pokemon Deoxys or Jirachi to complete your Pokedex. The Pokedex is cumulative, so you only have to capture one of each Pokemon once in the game. If you let the Pokemon go after you have captured it, the Pokemon will still appear in your Pokedex.
Talk to Professor Birch once you have completed the Pokedex. He will give you the option of choosing one of the Johto Starter Pokemon.
Connect a Game Boy Advance system running your game of "Pokemon LeafGreen" to the Game Boy Advance running "Pokemon Emerald." Trade the Johto Starter Pokemon from "Pokemon Emerald" to "Pokemon LeafGreen" by visiting the Pokemon Center in both games.
Using "Pokemon Colosseum"
Catch a Johto starter Pokemon in the wild. All three Johto starter Pokemon are available in "Pokemon Colosseum" in their evolved forms. There is an infinite number of these Pokemon, so you will eventually encounter and capture all three while playing through the game's storyline.
Connect your Game Boy Advance to the Nintendo Gamecube using the Game Boy Advance link cable. Make sure you have "Pokemon LeafGreen" version in the Game Boy Advance and turn it on.
Visit the Pokemon Center in "Pokemon Colosseum" to trade the Johto starter Pokemon over to your "Pokemon LeafGreen" game.
Using "Pokemon XD: Gale of Darkness"
Visit the Mt. Battle training area in "Pokemon XD: Gale of Darkness" and complete all ten challenge levels. Each challenge level has you facing 10 different Pokemon Trainers, so you will need to win 100 matches in total.
Select your choice of Johto Starter Pokemon as a prize for completing the Mt. Battle challenges. You are allowed to complete the Mt. Battle challenges as many times as you like, but you must repeat all ten challenge levels each time that you want to receive a Johto starter.
Connect your Game Boy Advance to the Nintendo Gamecube using the Game Boy Advance link cable. Put "Pokemon LeafGreen" in your Game Boy Advance and turn it on.
Visit the Pokemon Center in "Pokemon XD: Gale of Darkness" to transfer your Johto starters into your "Pokemon LeafGreen" game save.