We will make an announcement later this week or early next week, with all the requirements and links to appropriate guides/tutorials.
We'll post in the Scenario Design Forums, in Steam discussions and via newsletter so as many people know about this as possible.
As the files have a relatively small size we should be able to include several new designs for each region.
We will likely offer Crowns to the designer of any map we include in the game.
City design is a bit more complicated, but we'll look at what we can do to add in more customization.