Fire up WorldBuilder, and you get an uninspiring grey plain. Before we do anything about that, though, we need to make it the right size. In File, select Resize. Set it to 300x300 (multiples of 10 foot both ways), and leave the Border as it is. The player won’t be able to build on this area, but the landscape can’t just start in thin air in the middle of the mission area. Exactly how big everything should be depends primarily on how many players you’ll have.