Do you want to be able to edit the iDVD project at a later date? If so first open the project and use the File ➙ Archive Project menu option:
This will copy all of the media, photos, movies, etc., used in the project into the project file. That lets you copy the archived project file to a thumb drive for use by iDVD at a later date.
This screenshot shows what the contents of an archived project looks like:
If you want to archive a completed iDVD project use the File ➙ Save as Disk Image menu option and copy the results dmg file to the flash drive. This will produce a file that can only be played by DVD Player. No further editing can be done.