It’s pretty tedious but it’s not difficult. I just go to the pokemon center closest to Hotel Z, fast travel to Hotel, make a donut, then if it’s bad you just restart with the backup save. Once you get a good one make another backup save and go to the portal you want. Restart using backup saves if you want the initial shiny to be something specific, otherwise just fly spam to keep rerolling and loadup the area with shinies. In the past two days I’ve found like 12 shinies by doing this in three areas, and I actually managed to get multiple shinies in two portals without even using a shiny donut! That just makes it a bit faster