Hm. It needs some practice, for sure.
F5 is the tool and then you have 1 to x (don't remember now ) for the different options.
1 is add or remove ground
Left to add, right click to remove
Return key gives you a max height that you can add. This is especially helpful when you want a flat surface.
More wheel changes the texture type
Edit: with the G key you can switch between cubicle and sphere
2 is smoothing
Then there's water tool and an area tool, that let's you mark an area to add or remove.
I also achieved some good results in blueprinting a natural terrain, a hill for example, and adding it somewhere else, then smoothing the sides until it blends into the environment