Back up your iPhone to iCloud or a computer
Unpair your Apple Watch if you have one
Sign out of iCloud and Apple ID
Turn off Find My iPhone
Go to Settings
Tap General
Tap Transfer or Reset iPhone
Tap Erase All Content and Settings
Enter your passcode
Enter your Apple ID password if prompted
Confirm the erase
Wait for the iPhone to restart and erase completely
Remove the SIM card
Clean the device before trade-in