Do one of the following:
1. Search in the wild for a shiny Pokemon. There's a 1/8192 chance of finding one. You might want to soft reset for shiny starters or legendaries.
2. Breed the shiny Gyarados from the Lake of Rage (make sure it's male) with a female version of the Pokemon you want in shiny form. IIRC, there's a 1/64 chance of the baby being shiny.
3. Get the odd egg in Crystal. The Pokemon hatched has a high chance of being shiny.