Create a folder on your desktop called Pwnage
Download the latest version of RedSn0w from here and place it in the Pwnage folder. Likewise, download the firmware you would like to restore to and the firmware you are current on from here and place it/them in the Pwnage folder.
RedSn0w will now guide you through the steps to get into DFU mode. You can find more help with DFU mode here.
RedSn0w will check Cydia to see if you have the correct blobs saved.
If RedSn0w is unable to find your blobs automatically you can try to click the Remote button to try again or you can click the Local button to search for blobs on your computer.