The biggest thing about the terrain file is knowing what you're looking for in the existing ones... looking for examples of the syntax and structure...
Then it's really as easy as copying and pasting, setting the numbers to generate what you want. I have actually been amazed by the versatility of the terrain file once I understood it.
It's just a few banks of different types of assets, and a stack of layers, the order matters. The best practice is to go right down to the bottom, make your own layer, and drop a boundary circle, a height affector, a shader affector, and turn the flora and radials off with affectors set to remove all.
Then start trying to carve out differences within your flat circle, learn how to use the different boundaries shapes, and then start applying fractals and color ramps... then start trying to affect larger-scale changes. The first layer in the layers stack contains its own layer stack which is the general world shaping. The lower layers are usually for regional flora, and local landforms for points of interest, cities, or cave cutouts. So high in the stack is god stuff, the lower you go it's just little changes and niches.
In the Taanab file I'm working on, for example though, I went ahead and put some of my planet-wide affectors at the bottom because I didn't want to go back up into the unlabeled and unfinished mess, because I couldn't account for the contents of all of the layers in the middle, and I wasn't seeking to change the base terrain as much as the details . So it's not the most efficient way, but you can just override what's already there on a large scale even.
You can make your own flora, radial, and shader groups easily as well, assign them random family numbers and call on them in the layers... though it gets a bit tricky with the bytes in SIE, you will learn how to get what you want. You can string search for the affectors I talked about, and find them in action to see how and where they work. The ANH Wiki has a very complete breakdown that lists almost all of the available affectors, and a few that I have not seen used..