I recently installed the Co-authors plug in using my custom theme.
However, I'm not really sure how to implement it on a custom theme. He mentions using template tags, however I'm not really sure where to put those.
Here's an example of a current post with co-authors (but they're not showing up).
You'll see I also have a special box with my author in it- not sure if the Co-authors plug in can help with that by adding the other authors.