The syndication documentation gives a very basic beginning to creating an iTunes Podcast Feed, but leaves a lot of work left to be figured out. This is a completed example. Because I needed to obtain the duration of the podcast file programmatically (not part of this code) I limit the podcast to mp3 files. If you want to obtain durations some other way you can set the mime_type in the same manner as the other properties.
This is what I'm using and should be plenty for you to customize from.