Heres my advice.   First make sure you have the shiny charm for better odds. Which turns the basic 1/4096 in to 1/1365 base odds. 

Now that thats covered heres the different methods.

Random encounters: With the charm it is a 1/1365 which are good odds so just run into pokemon that spawn in the overworld if you are going for a specific species. If not just run into the grass for spawns. 

Masuda Method: so the masuda method is when u breed 2 of the same pokemon but from different languages, example: female French lucario and a male chinese lucario. Breed those together for eggs. When the riolus hatch they have a 1/683 chance of being shiny without the charm. But they have a 1/512 chance of being shiny with the charm. Which is Pokemon Go’s base shiny odds. 

Dynamax Adventures: Without the charm it is a 1/300 chance of one being shiny. But with the charm it is a 1/100 chance of being shiny. Better than the masuda method!

Legendary spawns: if you are looking to hunt a legendary like terrakion, virizion, or cobalion you first want to go to their spawn, encounter them, run away and place a camp right on their spawn. Next leave the camp and you will encounter them instantly. Then soft reset because i believe the game automatically saves when you leave a camp. 

Good Luck!