I have decided to make a tutorial to help people get better at terraforming. Step 1. Make a template. make a random structure out of cubes. If you don't know how simply type //sel cuboid This should be the default selector. Type //wand and a stick shall be in your inventory. Follow what the gui says. Step two. Select the sphere by //sel sphere or for designers do //sphere (item) (radius) After that select the two points and for constructors do //set (item). We will use this for a natural effect. Use preferably gravel or sand since they are gravity blocks and they stack but not float. This created a mound of gravel oh which we will later convert. (You can make the cubes or gravel sphere how big you want. Step 4. Now we need to change that gravel into a different material. (You can use any selector for this) And use the left/right to make a area that covers the mountain. Look for the two signs. Those are two places you could select. Then once you selected both points do //replace gravel stone This will change the gravel into stone allowing you to edit the mountain. Step 5. Fix the shape. You may notice that there is a bulk of gravel/stone In the shape of a sphere. We will need to fix this. To fix it simply break the stone/gravel into a smoother shape. After these steps you will have a neat shaped land form. If you use bigger dimensions for the spheres and cubes you can get something like this: This is what happened when I went crazy. I hope this helps!
making terraforming realistic should not come from coded commands, its all about going with the flow and just letting it happen naturally, but there are a few rules to make sure that it turns out nice. If you want to make giant mountains, yes this method can give you a mountain of sorts, but true terraforming will help you with all types of terraform and makes it look as realisticly 'random' as possible instead of it being completely down to commands to create something you call 'yours'. This tutorial is very useful, but i still believe letting yourself create the flow will overall improve the quality of everything you build. and one more thing when you make mountains for gods sake please dont use the 1 dirt block on top method. these are my opinions, but i know my brain doesnt work like everyone elses so going with the flow might not be your style
Edit: The screenshot of the massive landscape was a heavily edited/revised version of the tutorial. The world edit was used to create a template.