I know how to create a scene in-game by placing base structures, turrets, etc. But how would I be able to design the entire map from scratch, such as creating the dirt, trees, water, sky, gold, and the rest of the environment?

You have to make a new .RTE and draw out the terrain using paint or some other image editer, than code the scene. Look in Base.rte/Scenes for examples and you should figure out scene making pretty easily.

thnx