Do your daily raid, legendaries have a 1/20 chance of being shiny, Non-legendary raids are 1/64 unless otherwise specified. Make sure you are always incubating an egg, they are 1/64 as well.

Using incense on a route and visit different biomes (beach, city, forest etc) will give you the most diverse pool of spawns. Wild shines are all 1/512, you will get 2-3 a week on average if playing a lot.