1. Start from the installer USB
2. When you are at the CLI login as root, as usual.
3. Now create 2 new directories, used as mountpoints for accessing the packages:
4. Now we are mounting the second USB, that one with the ISO on it. To do so we have of course to know its device descriptor. You can check that with
I assume now that the device descriptor for that partition is /dev/sdc1 and mount that to USB:
mount /dev/sdc1 /usb
5. Now we mount the ISO to the /iso directory:
mount -o loop /usb/slackware64-14.0-install-dvd.iso /iso
6. Now you can just start the installer. When it comes to selection of the source for packages use the "Install from mounted filesystem" option (or similar, I don't know exactly how the option is named) and point the installer to the slackware directory under /iso.