Instructions to get iPhone DFU mode
Step 1: Open iTunes and make sure that your iPhone is connected to the computer.
Step 2: Press and hold the iPhone’s Power button and at the same time press the Home button.
You should press both the buttons together otherwise you will not be able to put your iPhone into DFU mode. If the Apple logo appears on your iPhone that means you didn’t press the buttons correctly and so the iPhone has loaded the operating system and started in normal mode.
Step 3: After 10 seconds, release the Power button and continue to press the Home button. You should keep the Home button pressed until a popup appears in the iTunes window that says, “iPhone detected in recovery mode". Press the OK button to continue.
After the iPhone has successfully entered DFU mode, you may notice either a black screen on your iPhone or a nice icon like a USB cable pointing to a hard drive. It depends if you have already jailbroken your iPhone or not.
Note: If you notice a blank screen on your iPhone after performing the above steps and you are still not aware if your iPhone is really into DFU mode, then you need to connect your iPhone with your computer. Start iTunes and wait for iTunes to detect your iPhone. After a while you may receive an alert from iTunes saying, “iPhone detected in recovery mode." You may then perform your required operation.
However, if you try to restore your iPhone while in DFU mode, you will get an error from iTunes along with an error code, which you will have to search for on Apple’s website to get a complete description. This error usually appears if you are trying to restore a jailbroken iPhone.
If you have any question(s) related to a blank screen on your iPhone, don’t hesitate to ask me.