Lesson 1: the background (bg) consists of 4 layers (0,1,2,3). You can stack bg tiles by placing tiles on different layers. Example, layer 0 can be then ground. Select 0 at the bottom and place grass tiles in an area. layer 1 can be trees, so select 1 and place trees. You can toggle which layers you see in the editor with the "0 1 2 3" on the top screen.