• Unlock bootloader (depending on vendor, you have to do an online verification),
  • flash a recovery.img,
  • load into recovery mode (which, depending on the phone, might need extra work)
  • wipe some caches,
  • select new os/rom image,
  • pray it doesn’t brick your phone.

You’d think someone would’ve learned a thing or two from the easy graphical installations linux and even windows have been offering since the late 2000s.