Additionally if you havent installed clonezilla to usb just mount your usb and then mount your iso of clonezilla, copy contents of iso to usb, then install syslinux
example assuming usb is sdc1, clonezilla iso is clonezilla-version.iso and assuming usb is not auto mounted and assuming clonezilla-version.iso is in /home/user_name/Downloads
Code:
mkdir /mnt/usb
mkdir /mnt/iso
mount /dev/sdc1 /mnt/usb
mount -o loop /home/user_name/Downloads/clonezilla-version.iso /mnt/iso
cp -r /mnt/iso/* /mnt/usb
syslinux -s /dev/sdc1
just copy and paste that into terminal as root you're good to go.
Note: if using syslinux in slackware it differs from one used in clonezilla live so you either need to install syslinux from a debian install or livecd/usb or manually enter the boot mode you want at error code prompt when booting.
Usually it'll say it cant find some lib32 something or it cant find vesamenu.c32
If it does that just view the syslinux.cfg/isolinux.cfg and enter entry you want to boot at prompt.
the newest Clonezilla default boot is
and it'll boot