Things You'll Need
Instructions
With Action Replay
Start a game as normal and choose any starter Pokémon. Save the game then shut it off.
Remove the ̶0;Pokémon FireRed Version̶1; cartridge from the Game Boy Advance and connect the Action Replay to the Game Boy Advance.
Slide the ̶0;Pokémon FireRed Version̶1; cartridge into the Action Replay. Power it on and select the ̶0;Code Entry̶1; screen.
Enter the master code: 72BC6DFB E9CA5465 A47FB2DC 1AF3CA86.
Enter the code for the two starter Pokémon you don̵7;t have. Bulbasaur̵7;s code is 8ED19163 3579E69D AD86124F 2823D8DA. Squirtle̵7;s code is 8ED19163 3579E69D BCDCB406 2F2FD763. Charmander̵7;s code is 8ED19163 3579E69D DB8C60CD 81FD6AA7.
Start the game and enter a grassy area to encounter and capture the two remaining starter Pokémon.
With Link Cable
Start a game and choose any starter Pokémon from Professor Oak.
Head north to Viridian City and go to the Poké Mart to collect a package for Professor Oak. Return to Pallet Town to deliver the package and receive your Pokédex and five Poké Balls.
Return to Viridian City. Along the way use a Poké Ball to capture any weak, throwaway Pokémon (your choices will be Ratatta or Pidgey.)
Enter the Pokémon Center in Viridian City and wait.
Repeat Steps 1 through 4 on your other copy of the game, except choose a different starter Pokémon.
Connect the two systems via the link cable and turn both on. Go upstairs in the Pokémon Center and talk to the lady behind the counter on the right to enable trading.
Trade a starter Pokémon for one of the throwaway Pokémon so that you have two starter Pokémon on one cartridge. This will be the ̶0;good̶1; copy of the game.
Save the ̶0;good̶1; copy of the game and set it aside. Start a new game on the other cartridge, select the only starter you haven̵7;t chosen yet, and play through to Viridian City. Enter the Pokémon center and trade it over to the ̶0;good̶1; copy of the game.