Ok, from the beginning ...
Right click on your finished sequence in the browser window (upper left) in FCE and select "Export" from the popup menu.
Pick "Quicktime" from the top of the sub-options under Export.
If you have enough disk space, select "make movie self contained", for settings use "Current Settings" (you don't need to recompress all frames), give your movie file a name, select a location with enough space to store about 15GB per hour of your final movie, and export.
(If you don't have enough disk space, uncheck "make movie self contained".)
Now, take the resulting movie file into iDVD (locate it in Media, or just drag + drop the file into the menu to make a button that links to that movie).
In iDVD, you can play the Movie to see how it looks. When you're done, either create a disk image or burn it to a DVD disk.
If you follow these steps, you DVD should be as good as your original unless your original was high definition.
Doug