what you need to do is just make your movie in iMovie with all the titles, music, and effects you want.
then click the share menu, and Export Movie...
export it however you wish, since your putting it on a dvd, you probably want to have it larger in size. then once that is done "sharing" open iDVD
pick your theme, create your titles and all that however you wish and when your ready to add the movie, click the media button on the bottom right, then click the Movies Tab and drag the movie you created in iMovie to your DVD.