Hunting random wild encounters: the simplest method there is.
Encounter hunting means meeting wild Pokémon over and over until a shiny appears. It works in any game and in any spot with random encounters: grass, caves, water and so on.
The game's base rate: 1 in 8,192 in Gen 2 to Gen 5 and 1 in 4,096 from Gen 6 on. The Shiny Charm improves this in most games that have it, but not in Brilliant Diamond and Shining Pearl, where it only helps breeding.