To start turning this patch of land into an actual place, pick a terraforming brush from the toolbar. The Height brush sets the area underneath to a fixed height; the Mound and Dig tools keep going each time you click the button. You’ll want a wide Brush Width when carving out the main details, and a wide Feather Width to keep things smooth. Just remember, the AI has to path-find across whatever you create. Here we have a rumpled terrain, with a riverbed.