For those of you seeking to install an upgraded or a downgraded version of iOS, this is NOT a "restore". Under redsn0w you do NOT use the Custom IPSW tab. I spent a frustrating week before I learned that you need to invoke the SHSH blobs tab in redsn0w, and find your stored blobs, then build a custom IPSW with the blobs imbedded. This is called "stitching" the blobs into the IPSW file. My everlasting thanks to a participant named Jaxov whose posting on a forum somewhere clued me in to this. If you do not have your SHSH blobs stored either locally or on Cydia you are out of luck here. You will probably not be able to go from 6.xx to 5.xx or from 4.xx up to 5.xx in iTunes. After creating a stitched IPSW file in redsn0w, you go the normal Pwned DFU route.