This isn't intended as a thorough guide to getting a shiny den. If you're looking for an extensive guide, consider using the one by /r/PokemonMaxRaids which is available here.
A hacked switch is NOT required to use this guide.
If you're just looking for the seed checker (シードチェック): https://cute.berichan.net/
- Make sure you have autosave turned off in settings, if you don't then you will lose your den as soon as you finish it.
- Put a wishing piece in the den you want shinies from. Use https://www.pokeatlas.com/ to take a look at what each den contains (Isle of armor can be toggled from the top bar)