Instructions
Defeat all eight gym leaders in Johto to receive all of the gym badges. Challenge and defeat the elite four to get access to the Kanto region.
Defeat all eight of the gym leaders in the Kanto region to receive all of the gym badges from this region.
Travel to Mt. Silver and go to the top to battle a character named "Red." Defeat him, then travel to Saffron City.
Go to the Pokecenter and place one of your Pokemon in storage so you have an open slot in your party.
Go to the Silph Co building and talk to Steven Stone, who is in the right hand side of the building.
Pick "Blue" when asked to choose a color and you will get a Mudkip.