It's all here in this post. I haven't done any code editing since the post. Some people above have pointed out above a few typo errors that still need to be fixed.

You might want to take a look at my post here if you haven't already:
http://www.ubercart.org/forum/development/3512/integration_postbooks_erp...

I want to "Normalize" the tables better than they are now. However, you might be able to get by with the functionality that is setup now and just adding the hook in to override the checkout forms.