Copying a Fedora install DVD ISO (e.g. The DVD image here), is the same as if you were directly copying over any ISO to USB. Note -- If you are trying to copy a LIVE image to USB, the directions in this answer will not work. You need to follow the LIVE USB directions on the wiki
There are two fairly easy ways to copy an ISO to USB, the first is using the GNOME Disks GUI, the second is using the dd command in the terminal.
Copying an ISO to USB using the GNOME Disks GUI
Insert your USB Drive, open up GNOME Disks, and choose the USB drive you just inserted from the list of disks on the left hand side. In this example, the USB Drive I inserted is the 8GB Lexar JC Firefly.

In the Volumes section in the main section, under the blue partition box, press the square icon to unmount the partition. (In this case, the USB drive has a FAT32 partition.)

Now the partition is unmounted, click the Button with the cogs on it in the top right corner of the window, and choose Restore Disk Image. Note that You will be prompted for root / superuser privileges at this step.

Next, choose the DVD install ISO you want to copy over, And choose the "Start Restoring button"

The image will now be copied to your USB drive. When the progress bar completes, you will be done! Eject the drive, and you will be all set!

Copying an ISO to USB using the dd command line tool
Alternatively, you can use the dd command to copy an ISO to a USB. For example:
sudo dd if=/home/username/Fedora-18-DVD.iso of=/dev/sdb
In this example, the if= parameter specifies the location of the ISO you want to copy. The of= parameter specifies the location of the device. Be sure to change both these parameters to the correct values before proceeding.
The easiest way to find out what the location of a particular device is to use GNOME Disks again. Simply insert your USB Drive, open up GNOME Disks, and choose the USB drive you just inserted from the list of disks on the left hand side. The location of the device should be listed towards the top of the right hand pane. In this example, the location of my USB drive is /dev/sdb
