• 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.