Adding a Director's Commentary

I'm looking to create a commentary track on a project for the first time and I
was hoping someone out there may have some ideas and/or shortcuts. I understand
that a director's commentary track is just like any other audio track, and is
inserted the same way another language track would be inserted in the Encore
project.

However, during playback, only one audio track is played... so it seems to me
that the director's commentary track needs to be mixed with the movie's regular
track... Is this correct? If so, how do you do this, adjusting levels as needed,
etc?