Step 1: Turn off your iphone
Step 2: connect it to itunes
Step 3: Press the power button for 3 seconds top button on off
Step 4: Without releasing power PULSS home for 10 seconds, you must have both buttons at the same time
Step 5: While holding home loose power and wait about 25 seconds, and you get your iphone into DFU, itunes mode will tell you it has detected an iPhone in DFU mode.
regards