You can use the lightning-to-USB cable that your phone came shipped with.

Once you press down on the side button, start counting to 10 as you will continue to the next step and add another button.

After you’ve pressed down the “Volume Down” button, start counting to five.

If the Apple logo appears, you held the side button down too long and will need to try again. The screen of your iPhone will remain black and display nothing when it’s in DFU mode. The only way to tell it’s in DFU mode is that “Restore iPhone” is the only option available in iTunes.