If you take a peek in that flash-all script you can see exactly what it's doing. Based on my memory, it: Flashes the bootloader image. Reboots. Flashes the recovery image. Reboots. Flashes the modem/radio image. Reboots. Flashes system, userdata, and cache images all at once. Reboots. So you still need to do the fastboot oem unlock step (to unlock the bootloader), but you can otherwise just execute Flash-All and call it a day*. * Unless something fails to flash, which seems to be a commoner and commoner thing lately. You'll then likely need to flash the filesystems individually as described at the top of this thread.