How to get into DFU mode
How to put your iPhone into DFU mode:
Step A: With iTunes open and your iPhone connected.
Step B: Press and hold the iPhone’s Home button and Power (Sleep/Wake) button and hold both for 10 seconds.
Step C: After exactly 10 seconds release the Power (Sleep/Wake) button. continue holding the home button until iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
After the iPhone has successfully entered DFU mode, your iPhone will be black like this:
Note: It may take a few attempts to get your iPhone into DFU mode. Generally, hold down both buttons then release the Home button just before you think the Apple logo would appear. If you are still holding both buttons down and you see the Apple logo you are holding them down for too long!