How to put your iPhone into DFU mode.

DFU means Device Firmware Update. If a restore using Recovery Mode doesn't work you will want to use DFU Mode as a last resort. When placing your iPhone into DFU Mode it does not load the OS before attempting the restore


Step One
Open iTunes and connect the iPhone to your Mac.


Step Two
Press and hold the Home button and the Sleep/Wake button at the same time.


Step Three
After exactly 10 seconds release the Sleep/Wake button. Continue holding the home button until you iTunes pops up a message telling you that it has detected an iPhone in recovery mode.


The iPhone screen will remain black like this:






Thanks to iclarifield