Make your musical project time line active. Go to Share and Export Media current settings. Do the same for your behind the scenes project. That will give you two 1080p25
files that you can then import into iDVD. Within iDVD make sure your project is set to 16:9, you should be able to create a DVD (SD, of course) with very good image quality and in the correct aspect ratio.
If you want to chapter markers, you need to add them in another application like Compressor or Metadata Hootenany. (If you do this in Compressor, do not encode to a different preset…keep the size and codec the same and just add chapter markers.)
Good luck.
Russ