Check the "Dumping/Using SHSHs with iFaith" section. (The "Restore SHSHs with iFaith" part, more specifically.)
Unfortunately, redsn0w still needs to updating to support stitching for 6.1.2, and iFaith for Mac isn't available (yet), so Mac users need to find a Windows computer.
Unfortunately, redsn0w still needs to updating to support stitching for 6.1.2, and iFaith for Mac isn't available (yet), so Mac users need to find a Windows computer.