How to put an iPhone into DFU mode
Step 1: Plug your iPhone into iTunes
Step 2: Press and hold the Home and Sleep/Wake buttons at the same time.
Step 3: Count to 10. After exactly 10 seconds let go of the Sleep/Wake button, while still holding the home button.
iTunes should pop up with a message telling you that it has detected an iPhone in recovery mode. (iTunes will not distinguish between DFU/recovery mode)
NOTE: The screen on your iDevice should remain black. If you see an iTunes logo on your device, then you are in Recovery mode and not DFU mode.
NOTE 2: It make take a few tries to get the device into DFU mode. A tip – If you are still holding both buttons down and you already see the Apple logo appear, you held them for too long. Start again until the screen remains black, and iTunes tells you it has detected a device in recovery mode.