Though there's no guaranteed way to get a Shiny. However, chain fishing is the only easy way when it comes to Shiny hunting.
How to Chain Fish-
-Get a fishing rod and a Pokemon with Sticky Hold or Suction Cups
-Put said Pokemon at the front of your party
-without shutting the game off or moving from your place, keep fishing in the same place over and over again until you get a Shiny
You can switch rods, just as long as you reel it in every single time. Shutting the game off, moving from your position, or failing to reel it in stops your chain. You can run from the Pokemon, so you don't have to kill them.
If you don't want to do this, the best option is to run into grass and use sweet scent nonstop in the overworld and keep finding hordes until you get a Shiny.