How do I fix this brick and get the phone back up and running? I'd greatly appreciate the steps laid out in plain English a newbie would be able to comprehend easily enough. Thanks
You can't downgrade from the stock 4.3 bootloader to an older bootloader. Trying to flash an older bootloader will brick the phone.
You need to be on stock 4.1.1 in order to use the free unlock method. A custom 4.1.1-based ROM will not have the required menu options.
Thanks. Maybe I didn't phrase my question properly ... I mean is it possible to go to from stock 4.3 to a custom ROM based on 4.3 (or newer) e.g. CM11? I understand you can't downgrade from stock 4.3 to an older bootloader which is why I'm asking if a workaround involving stock 4.3 to custom 4.3 based and then back to stock 4.1.1 is possible? I don't know if anyone has been able to successfully flash custom 4.3 based to the AT&T version of the GS3 running stock 4.3.