Go to applications in the finder, and go to utilities. Choose Disc utility
Put in the dvd, when it shows up in the disc utility, go to file, new, disc image (choose the disk, I think mine is something like disc1) Under the image format, choose dvd/cd master.
Pop out the DVD, and put in a blank, to burn choose the dvd image, and click the burn button.
I'm doing this from memory, so you may need to look at the help files if I missed something.