Basically you plug the new HDD into a spare SATA port on your PC, and use WinMFS v9.3g (in Windows) to 'restore' an image to your new HDD. Then put it in the TiVo, and do a 'clear and delete all'.
Before removing the new HDD from your PC, you could also run WDIDLE3 to disable intellipark, to avoid having to cycle power on your TiVo when it reboots.