Part 2
How do I avoid failing shinies?
SAVE THE GAME!!!
If you care about keeping the shinies you find, then it's always a good idea to save whenever you see a shiny. If you know a shiny has a self damaging move, you can run away from the battle and save. If you manage to KO the shiny or it KOs itself, you can always reset the game to get it back, assuming you have Autosave turned off. Sometimes, it may not appear right away, but if you reset again, it should show up.
Do basic research and come prepared
A mistake many inexperienced shiny hunters make is going into a battle, weakening the shiny, and the shiny KOs itself. While saving prevents losing the shiny, knowing what moves a Pokemon has and properly preparing is still valuable, especially to save sandwich time. In general, prepare a Pokemon with False Swipe, a status inflicting move (preferably sleep) moves like Taunt, and even bring abilities like Damp, depending on your target.
Keep an eye out for shinies
In general, knowing what a shiny looks like is the best way to avoid failing it. Scarlet and Violet have their fair share of subtle shinies that you can easily miss. Your Pokémon will always refuse to auto battle a shiny, so if you see a Pokémon and you aren't sure if it is shiny, target it with an auto battle just to be sure.
Sometimes, Pokémon spawn in the walls. Usually, these are a lost cause, since you can't target Pokémon found in the walls. However, if the shiny gets close enough to the wall, you may be able to encounter it. If you are playing on the base version, you can lock on to the Pokémon with ZL and throw a Poke Ball at it if it's close enough. In general, it's best to avoid walls with Pokémon spawning in them.
Be careful with despawning Pokémon!
Setting up a picnic and entering certain zones like cities and towns despawn all Pokémon. This is a useful technique for going through encounters quickly, especially in places like caves. Just be sure to make sure there are no shinies before setting up a picnic or entering a despawning zone or you might lose your target.
Don't run towards the camera! Pokémon tend to spawn both around around you and wherever the camera is focused. So if you are moving towards the camera, there's a chance you can spawn and despawn a shiny at the back of the screen before you can even reach it.
Any Pokémon you run into in a Union Circle will despawn when you run away from the battle. If you or someone else disconnects from the Union Circle, all the Pokémon around you will disappear. The only exception to this is if you save in front of Pokémon you spawned, they will reappear if you reload the game to your save point. All other spawns won't reload if you didn't spawn them. If you know a shiny is your own spawn, it's best to save just in case something happens.
Zorua, Zoroark, and Ditto disguise themselves as other Pokemon in the overworld. If you run into them, and run away from the battle, they will despawn. To hunt them, make a Dark/Normal encounter Power sandwich, auto battle all imposters, and when your Pokémon refuses to battle one, save. The imposters will have indicator that they are a disguise. If you lock onto the Pokémon with ZL, you will see their real name even though they are disguised. If you get close to them, they will start to sweat and run away. If a Pokémon looks like it's walking suspiciously, it is likely a disguised/transformed Pokémon
Shiny Dondozo outside of an outbreak can despawn itself. Shiny Pokemon usually have a mechanic implemented where if the regular version can despawn, the shiny has a slightly different mechanic so that it doesn't despawn. However, if a Dondozo dives and is left alone for a few seconds, it will despawn regardless of if it is shiny.
If you encounter a shiny Pokémon while in Synchro Mode, don't run into it as you will KO it. Try to lure it back to your character so that it won't despawn Once you exit Synchro Mode. Once it's close enough, you can exit Synchro Mode and encounter it with your trainer.
(Note: I have not played around with this myself, but I think you can only scare shinies back to your trainer with certain Pokémon.)
Although not technically despawning a Pokemon, if you connect to the Internet while you have old event Outbreaks/Tera Raids in your game or when there are new events available, your current raids and outbreaks will reset. This means that if you find a shiny in a Tera Raid and accidentally connect to the Internet during this period, you will lose the Tera Raid and the shiny. Be careful if you're trying to share a shiny Tera Raid with others online.
Other Miscellaneous Tips and Tricks
Holding ZL and targeting a Pokemon will allow you to throw your ball at it. This is useful for Pokemon that are flying, deep underwater, etc.
Additionally, on the base version, if you hold ZL and hold your right control stick at a certain angle, you can see the spawns under the water. This is useful for hunting Pokémon like Skrelp. However, the camera acts different in the current version, so I don't know if this is possible anymore.
On the Switch 1 version, up to 15 Pokémon can be present at a time, but if there is a shiny, there will be 16 Pokémon present. This is useful for hard to notice shines like Tandemaus and is likely the best method for hunting Minior in the wild, as you cannot visibly tell it is shiny unless it is in its Shield's Down form. So if you see 16 Pokémon on screen, save and check each one.
If you adjust the color settings on your TV, you can make the blue and tan pop out more on Tandemaus, making the shiny a lot easier to see compared to the original.
If there are two shinies on opposite ends of the screen and you want both, save, then reset until the Pokémon start heading towards each other or until you can reach one without despawning the other. With patience, it is possible to keep them both.
Look out for event Outbreaks/Tera Raid events! Some event outbreaks allow you to easily find shinies that are hard to find otherwise, and some event outbreaks have boosted shiny odds! Aside from a previous shiny Rayquaza Tera Raid event, there haven't been any Tera Raids with boosted shiny odds. However, Gimmighoul, which is normally shiny locked in normal gameplay, has previously appeared in event Tera Raids with it being unlocked in 5 star raids.
The Switch 2
The Switch 2 patch has brought about many quality of life changes to improve the gameplay. The Switch 2 runs smoother than the Switch 1, making certain areas in the game like Tagtree Thicket and Casseroya Lake better for hunting in as they suffer from lag and frame rate drops far less frequent and intense than in the Switch 1. However, the most significant change by far is the amount of spawns being changed from 15 to around 40, allowing for you to get far more encounters and therefore, more shinies. In addition, Pokémon can spawn in from far away and you need to move much farther away from them to despawn, so combined with the quicker picnic resets, despawning Pokémon and finding your shiny will be much quicker and efficient than before.
Some negatives of this are that it is more likely you will miss a shiny that you simply do not see, and hunts like Minior that relied on the Switch 1's number of spawns are now more efficient to do on the Switch 1, but overall, the Switch 2 patch seems to be a net positive for making shiny hunting even quicker.
(Do note that I do not have a Switch 2 myself, these notes are from my observations of what I've seen and heard online.)
Let me know if there's anything I missed and happy hunting!