There’s less mashing compared to Sun & Moon, but it still goes slow. In Ultra Sun & Ultra Moon, you’ll want to save right before entering the grass on Route 1.You can check from the nickname screen, but be careful, because Shiny Popplio is not too noticeable. Unfortunately, there is a lot of text mashing you’ll have to go through before you can check if your starter is Shiny. Optimally, you’ll want to save just a step away from the exit.
Some games also have an L = A button mode, which can let you mash text using the L button as well and can free up your right hand to check Twitter, change the channel, etc. Start up the game, make sure you turn Text Speed to “Fast” as soon as you’re able.(Make sure you erase your save file before starting the New Game, otherwise you won’t be able to save!)
This is obviously something that cannot be done on an existing game. Regardless of which game you’re playing, here’s the basic process for hunting for a Shiny Starter Pokémon: The Shiny Charm does not apply here as you cannot legitimately receive the Shiny Charm before obtaining your first Pokémon, silly. Your odds are a fixed 1 in 8192 in Generation II–V games and 1 in 4096 in Generation VI–VII games. This is generally impractical if your goal is to get the starter Pokémon by any means, as breeding will go faster in most games, but there’s something to be said about having your first Pokémon in a game be a Shiny Pokémon, which is why many people enjoy taking on this challenge.
In every single main series Pokémon game - with the exception of Pokémon Colosseum, Pokémon XD, Pokémon Let’s Go! Pikachu & Eevee, and Pokémon Sword & Shield - you can attempt to get a Shiny Pokémon as your starter Pokémon.