Instructions
Start a new game. Name your player character and rival and you will begin the game in your house. Exit your home and proceed north out of Pallet Town and into the grassy fields.
Speak with Professor Oak who takes you to his lab and asks you to choose a starting Pokemon. You can select between Bulbasaur, Squirtle, or Charmander. Do not choose a Pokemon yet.
Save your game and choose the Pokemon you want. Your rival will choose his and you'll enter your battle.
Check to see if your Pokemon is surrounded by stars at the start of the battle. If this is the case, you have a Shiny Pokemon. If not, hold "A," "B," "Start" and "Select" to reset the game.
Load your saved data to return to the selection process and pick again. Repeat this until you have obtained a Shiny Pokemon.
Save your game as soon as you end your first battle with your Shiny Pokemon. This will ensure that the next time you load your game you will still have your rare find.