Extract the RedSn0w zip file into the same folder.
Step Two Connect your iPhone to the computer and launch iTunes.
Select your iPhone from the list of devices on the left. Now hold down Shift and click the Restore button. Restore is preferred as it won't create any wasted space on your iPhone.
Navigate to the Pwnage folder on your desktop and select the 4.3.1 firmware ipsw. Click the Choose button to continue.
Step Three Once iTunes has finished updating your iPhone to the desired firmware open the Pwnage folder on your desktop and launch the redsn0w application from the redsn0w folder we extracted earlier.
Step Four Once RedSn0w opens click the Browse button
Step Five Select the 4.3.1 firmware ipsw we placed in the Pwnage folder on our desktop then click Open.
Step Six Once the firmware has been verified click the Next button to continue.
Step Seven RedSn0w will now prepare the jailbreak data
Step Eight From this window you can select the jailbreak options you would like.
Make sure Cydia is selected and click the Next button to continue.
Step Nine Please plug your iPhone into the computer and make sure its OFF then click the Next button
Step Ten RedSn0w will now guide you through the steps to get into DFU mode. You can find more help with DFU mode here
Hold down both the Home button and the Power button for 10 seconds.
Release the Power button and continue holding the Home button until RedSn0w detects the device.
Step Eleven Your iPhone will now reboot
Step Twelve RedSn0w will then begin uploading the new RAM Disk and Kernel.
Step Thirteen Once this is complete you will be notified that RedSn0w is done. Click the Finish button. When your iPhone finishes rebooting (5 minutes or so) it will be jailbroken with Cydia on the SpringBoard.
*As usual a big thanks to the iPhone Dev-Team and I0n1c for their hard work and contribution to the iPhone community. **Windows 7 users may want to try running the redsn0w executable mode as an administrator in Windows Vista compatibility mode if you experience any trouble.