Follow my steps:

1) Make sure your iphone is connected to your PC, and iTunes detected and launched

2) Now, switch off your iPhone while being connected to the PC (iTunes must remain open)

3) Hold the Power button + Home Button, after 10 secs release the Power button but hold the Home Button ( your PC should detect your in DFU mode ). Your iPhone screen will be BLANK ( if you get the cable + iTunes logo, it means you held it too long and went into recovery mode)