If you're looking to make a bootable USB installation
media, something like the Universal USB Installer utility might work: http://www.pendrivelinux.com/univers...easy-as-1-2-3/
Better still, the official instructions on doing this from Red Hat themselves:
However, if you're wanting to boot into a live environment of sorts... I'm not sure RHEL6.x supports a live environment running off a USB stick. Indeed, looking further down in the docs linked at RH above it appears they only support making Live USB sticks using RHEL7, not 6.
For what it's worth, I know Centos7 has a live environment image you can download and install to a USB stick. Further, using the liveusb-creator within Fedora allows you to make a bootable USB stick with persistence--that is, you can make and save changes that remain after rebooting etc.
So, depending on your ultimate end goal, if you're looking for a persistent USB-based environment you'll probably need to look into either Centos7/RHEL7 or Fedora instead.
Hope this helps.