there a number of guides on howto create bootable usb sticks.
I am not sure rhel is the ideal candidate...I think slax for usb is the easiest.
but in summary..for a single partition usb stick
get your media...mount it...it could be your existing sytem but that may be too large
create a new dir
cp the contents of the mounted iso into new dir
install a bootloader into usb mbr
and usb stick should have a bootable flag
umount usb and mounted iso
change bios to allow detection of usb hd...(not all bios allow it)
reboot to usb to test
I am suggesting again try slax for usb first
yes it uses lilo but you can make it grub