Yes - definitely draw up an outline plan. What do you want shown? What is the theme, the aim, the shape of the festival? Can you realistically acquire screening rights and the goods? Which venues? Who owns them? - approach them. Think about how you will acquire the necessary capital - and of course you'll have to work out how much it'll all cost, first. Structure in marketing costs, once you've worked out how you're going to promote it. Think about potential sponsors - even if you need to run it by the council, and they agree, and they even stump up some funds, it won't be enough.
Very, very focused thinking is needed.