Alright. Here's a tutorial on Terrain editing written but an Expert in the matter. This is for SFS 1.4 but still applies in 1.5: https://jmnet.one/sfs/forum/index.php?threads/lesson-1-3-terrain-data.3400/
Once again, notice what I said about that you will have to experiment a lot by yourself to obtain that shape you want. Not experts know clearly how to obtain that shape.