Yeah the advice all seems pretty vague and not to focused.
How I see "Build a Community" is really build a following of people who love your product and support you.
How to do that is the tricky question, but the major thing is to get your idea and product out there...if no one knows about it, how are they going to back you.
To get your idea out there, yes it's forums, email pitches, websites, FB communities and being part of the niche groups you want to target. If you come across as being an expert in your field, then people are more likely to trust you and support your potential campaign.
Hope this little bit of advice helps. Are you launching a campaign?