Instructions
Save the game before choosing your starter Pokemon. The game will give you a choice of starters, with Charmander being one of the options. You must select the Charmander and look at the status screen to determine if it is shiny. Reset the game if it isn't and try again.
Save the game when you encounter a Charmander on the map, before you start the battle. The stats and shiny status are determined when the battle starts. If the Charmander is not black, reset the game.
Breed two normal Charizard at the nursery on the fourth island. The shiny factor is determined when you pick up the egg, not when the egg hatches. Save your game before you pick up the egg, incubate it and see what comes out. Reload the game if it is not shiny.