The easiest way is ultra wormholes, but they are SO common in ultra wormholes that they aren’t really considered valuable. The best way in my opinion is to S.O.S Chain.
To S.O.S chain, you have to get a Pokémon that can call for help to low HP (use False Swipe), use an adrenaline orb, and then defeat every called Pokémon until you find a shiny. The chances are increased to 1/1024 from a chain of 70-255, and a 1/683 chance with the shiny charm. The call rate of a Pokémon determines how often it will call for help.
0 call rate:Never Calls
3 Call rate:Rarely Calls
6 Call rate:Sometimes Calls
9 Call rate:Usually Calls
15 Call rate:Almost Always Calls
The call rates are here: https://m.bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_by_call_rate
There is the Masuda Method which involves hatching eggs with the parents being from different countries, and the chances are 1/682 with no shiny charm and 1/512 with shiny charm. The chances are a little better for the masuda method, but you can chain way faster than you can hatch an egg. I have gotten many shinies from S.O.S chaining(Zoroark, salamence, braviary, crobat, midday and midnight lycanroc, clawitzer, tyranitar, milotic, flygon, and gigalith) and I know it is an effective method. I suggest chaining Pokémon at a 9 or higher call rate, and 6 if you are patient. You can also get HA Pokémon from S.O.S chaining, and a few of my shinies have hidden abilities. A good way to S.O.S chain is to get a Pokémon with Harvest and Skill Swap(Exeggutor or trevenant) and a Pokémon with trick and a leppa berry. If you skill swap the calling Pokémon harvest and trick it a leppa berry, they will never run out of PP so you never have to switch callers.
Hope this helped!