Ok, one trick is to start with a perfectly flat terrain. First, start by dropping any old terrain into your scene, and then right-click on it to Edit it. When you're in the terrain editor, click on the Reset button. This will flatten it out completely. Then, select the detail size you really want (maybe 512 instead of the default 256). Then use the painter to Raise the area for the big oak tree, or such. For what you are doing, it may be easier to always start with a perfectly flat terrain, and then paint onto it your landscape as you will.