Be sure and save them where they're easily accessable... make a folder on your desktop and put them both in it. Also... be sure to download the IPSW file using a browser other than Internet Explorer. Use Google Chrome, Firefox, Safari... anything but IE. Once you have both redsn0w and your IPSW downloaded we'll get you going in the right direction.
You'll right click on the redsn0w.exe file and "Run as Administrator." Then choose the option for pwned DFU mode... you'll need your iPhone connected to the computer for this, of course. Once the pwned DFU mode process completes... you'll open iTunes (stop the Auto-sync if you have it enabled), hold Shift and click "Restore." This will bring up and Explorer box... you'll navigate to the 6.0 IPSW you have saved from just a bit ago. This will directly restore to the clean IPSW w/ your iDevice in pwned DFU mode.