Insert the DVD and connect the USB drive. Launch the Disk Utility application. If you haven't already done so, partition the USB drive in GUID Partition Table format, with a single partition in Mac OS X Extended (Journaled) format. Select the Restore tab. Drag the icon of the DVD from the left pane of the DU window into the "Source" field. Drag the icon of the volume you created on the USB drive to the Destination field. If you already have data on the drive, select the "Erase destination" checkbox. The existing data on the drive will be lost. Click Restore. When the operation is done, boot from the USB drive to test it.