You have to install dvdauthor. Next make an dvdauthor config file that will include titles for each seperate movie. Though not all dvd players will handle a custom bitrate, gop size, size of the video. You may have to use ffmpeg to re-encode to a DVD format.
I do not recommend using GUI programs to help you with dvdauthor because they are not reliable for me. The man or info pages gives you some examples of what you can add in a dvdauthor config file. Use xine to test the final DVD output before making a ISO file.