This assumes that the capacity your USB drive is no less than 4GB.
- Install grub4dos to the USB.
Use grubinst to install grub4dos MBR to the USB, then copy the grldr file in grub4dos package to the root directory of the USB drive. See DoIT’s post for instructions.
- Put Installation media to the USB
Download the fedora DVD iso image, extract vmlinuz and initrd.img in the isolinux directory to the root of the USB drive, and then put the iso itself to the root of the USB drive
- Boot from the USB
Boot from the USB, you will be dropped into a grub shell, type the following command to start the install.
root (hd0,0) kernel /vmlinuz askmethod initrd /initrd.img boot
The will start the fedora installer, when it comes to the install method selection, select Local drive, and select your usb drive partition to continue.
Discussion: Why not simply use UNetbootin? 1st, Fedora 15 is not yet listed on the homepage, maybe it is not supported yet. 2nd, the most important reason, UNetbootin will copy the contents of the ISO image file by file to USB, which is quite slow. If drop the iso image itself to USB, it will be much faster.
Reference: Official Fedora 15 Installation Guide