It is recommended by Apple to charge an iPhone with the official Apple brand iPhone charger. When preparing the phone for charging, it is recommended to insert the end of the charger directly into the charging dock of the phone, avoiding elevation of the phone to prevent breaking or putting too much stress on the charging cord.
The USB charger should be inserted directly into a power adapter that is plugged into an outlet. Another option is to insert the USB charger, connected to the phone, into the USB outlet on a computer. If a docking station designed for the iPhone is available, that will charge the phone sufficiently, too, according to Apple Support.