To enter DFU mode with the iPhone X , you have first to plug the iPhone X to a computer in order to connect via iTunes using the ligthning cable, and then set the place to shut down, press and hold the “side button” for about three seconds after the shutdown, iPhone did not show any picture, but iTunes has shown a Only the iPhone in the reply mode on your behalf successfully entered the DFU mode.
To get started First you need to :
Step 1: Update to the latest iTunes
Step 2: Connect your iPhone X to your computer PC or Mac using the lightning cable
Step 3: Force restart your iPhone X by pressing down and holding the volume up button, then the volume down, then press side button all together until you see a reboot.
Step 4: In iTunes, you will have two options to either Restore or Update. Click on the Update, iTunes will restore your iPhone X / iPhone 8 to the latest iOS software without loosing any data.
Note: In case you want to disconnect your iPhone from recovery mode, unplug the iPhone X from your PC or Mac, press the side button to start your iPhone x. Also while doing this process, if you get disconnect, you have to go back and do Step 4 again.