Create a new folder on your desktop called Pwnage.
In the folder place the most recent firmware (which can be downloaded from here: iPhone, iPad, iPod Touch) and the latest version of RedSn0w (from here).
Extract the RedSn0w zip file by double clicking it.
Launch the redsn0w application from the redsn0w folder we extracted earlier.
Once RedSn0w opens click the Extras button.
Choose Even More from the Extras menu.
Select DFU IPSW from the Even More menu.
You will be instructed to select an IPSW that you can currently restore to without any hacks or pwnage steps.
Select the ipsw firmware file you downloaded in Step One and click Open.
RedSn0w will now proceed to create a DFU Mode IPSW.
You will be notified once the IPSW has been created successfully. Click the OK button.
Connect your device to the computer and select it from list of devices on the left. If you haven't performed a backup recently, perform one now. Next, make sure you are on the Summary then hold down the Option key and click Restore.
Select the ENTER_DFU ipsw from the Pwnage folder on your desktop and click the Open button.
After your device has been restored it will be dumped into DFU mode. The screen will remain black and you may be able to jailbreak depending on the firmware you've restored to.
Check out the iClarified Jailbreak Wizard for more information.