Use a USB-C cable to connect one iPhone to another if both support USB-C and power sharing
Use a Lightning cable with a power source connected to one iPhone only if the other iPhone is being charged through an accessory or adapter
Use MagSafe or wireless charging with one iPhone placed on a compatible charging accessory powered by the other iPhone through a battery pack or adapter
Use a battery case or portable power bank charged by one iPhone, then charge the other iPhone from it
Use a USB-C to Lightning or USB-C to USB-C cable with a compatible iPhone model that supports reverse power sharing
Make sure the source iPhone has enough battery before attempting to charge another iPhone
Keep both devices unlocked if prompted to allow accessory or power sharing
Check that the cable and charging ports are clean and properly connected