HOW CAN I MAKE SURE MY OLD IPHONE MEMORY IS COMPLETELY DELETED BEFORE SELLING IT?

I wish to sell my initial era iPhone, but wish to have certain it has been utterly spotless of my personal data. I already privileged the mental recall regulating the “Erase all Content and Settings” button, but I’ve listened which this should be finished mixed times to be safe. However, once you do it the initial time, the phone requires you to go by set-up again to get behind to the desktop where the settings entrance idol is located. What should I do?