I think that making a full functioning forum is something that would require a bit of planning up front. If you would like to make one from scratch I think that you would need to look at it like a new web app that you are creating. You would start with a set of specs then based on that come up with a strategy for the user experience. Once you know how you would like your user experience to be you can then start to develop the site with those goals in mind.
I think this would be the best approach, I know from experience the larger the project is the more this type of organization will help you get it accomplished. Knowing concretely how you would like it to look and work will help you to be able to focus on the development task at hand rather than trying to figure it all out as you go.
This is not a project that we have any guides on and the system we use on our forums is not one that we developed ourselves so it will be very free form as far as getting together all of the parts.
