This is how I'd do it on Slackware-14.1:
isohybrid -u slackware64-14.1-install-dvd.iso
# plug in the USB stick
lsblk -o NAME,MODEL,SIZE,TYPE|grep disk # Check carefully the results
dd if=slackware64-14.1-install-dvd.iso of=/dev/sdx bs=4M; sync
- isohybrid -u makes the ISO image EFI bootable. Won't work for big ISO images if Slackware version is less than 14.1
- replace sdx in /dev/sdx by the relevant device name read in output of the lsblk command. Be careful not to chose a hard disk, that would wipe it out.
- save all data on your USB stick before proceeding as all will be erased.
Other methods are proposed e.g. on this page
. On Windows I recommend using Rufus
PS Sorry, I think that I misunderstood your question. You want to install Slackware on the USB drive itself, not use it as a bootable media to install Slackware elsewhere, right?
Then, just plug in the USB stick before or just after you boot the installer (Slackware64-14.1 only).
Just follow the instructions. You'll have to make an EFI partition of 100M in addition to the ext4 one on the USB stick. I suggest that you designate the / (and/or EFI?) partition by LABEL or UUID in /etc/elilo.conf and /etc/fstab as the device name can change upon the order in which removable devices are plugged in. Unfortunately I never did that with elilo as I don't have an EFI firmware, just guessing that's similar as doing it with lilo.