From Fedora Project Wiki
Making a USB key of Fedora
You are probably looking for this document, which tells you step-by-step how to create a Live USB key of Fedora using a pre-existing ISO file you downloaded from somewhere (probably spins.fedoraproject.org or get.fedoraproject.org):
How_to_create_and_use_Live_USB
Making your own custom USB IMAGE of Fedora
You may be interested in creating your own custom ISO from scratch. If so, you'll want to read this page:
How_to_create_and_use_a_Live_CD
Some documentation on kickstart might be handy too:
by the way, if you have an x86_64 machine you're building on but you want a 32-bit happy iso image, add the following before your livecd-creator command:
setarch i686 livecd-creator
Good luck!