Back up your iPhone to iCloud or a computer
Make sure the backup completed successfully
Sign out of iCloud
Turn off Find My iPhone
Sign out of the App Store and other Apple services if needed
Unpair your Apple Watch if you have one
Remove your SIM card
Erase all content and settings
Remove your iPhone from your Apple ID device list
Clean the iPhone and remove any case or accessories
Gather the original box, charger, and accessories if required by the trade-in program
Check that the device powers on and is ready for inspection if needed