it depends how long your dvd is?
If it's not too long then you can easily do this using something like sony vegas software.
All you do is load in the video, then add a new audio track and put in your commentary where you like it.
The problem I have found is that you have to then render the film which takes about a minute per minute of film.
Then what i've found best is get the imtoo dvd suite which will then let you change the file type to anything you want..it can even let you do it in a format for your mobile phone!
Hope this helps