Step 1)

Open iTunes and connect the iPhone.

Step 2)

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

Step 3)

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.

Step 4)

The iPhone screen will remain black. Also bear in mind, that in some cases it may take a few attempts to get your iPhone into DFU mode.