If you want flat terrain (like really flat), then you can simply use the :FillBlock() function. Create a part and size and position it as you desire, name it something unique, and use the command bar line to use the function.
workspace.Terrain:FillBlock(workspace.PartCreated.CFrame, workspace.PartCreated.Size, Enum.Material(YourMaterial)
Check this out, it covers basic terrain scripting.
Not flat terrain, I mean a way to make the above smoother. This is it’s side view. (Look in the upper area of this image)
If you zoom in it appears bumpy, which i dont want. I want smooth curves like in the above photo
I did use hightmap to import this btw
What do you mean it is not very effective? Smooth tool should work. As for the size problem, you just have to be patient with terrain building. It can be time consuming, but the end result is worth it.
As an alternative, you can also use the grow tool at a fairly weak strength and with normal to large size. Other than that, I just don’t really know what can help with smoothing.
Well it just doesn’t do what it’s supposed to do - smoothen it out. I literally have no idea what it does but its not affecting my terrain in any way.
I assume you mean the smooth tool … ?
If so, try turning up the strength of the brush. It should do something 
.
Also, take a look at this video, this shows that the Smooth Tool does it’s job.
Skip to 0:52.
Edit: Video from here.
That’s the issue it’s not working like shown in the video above. Strength plus size is on max
Any more help? It’s very annoying…
That’s not something your doing, it’s just how smooth terrain works. It consists of voxels which are pretty big, making it bumpy, also another factor is ambient occlusion, if your talking about the black shading/outlines. That only gets turned on at higher graphics.
What I would do is redo the terrain using a 3D modeling software called blender
I have used blender but I don’t like to use it since the collisions on it are buggy. Guess I have to find a way to make it smoother. I’ll try the occlusion though 
Yeah that can be avoided but I get what you mean, there’s no chance of getting smooth terrain to get smoother tho sadly.
Well it’s not really smooth as in the picture above but ok
There was a similar thread, I think with a solution. Let me try and find it.
So the thread I remembered was for terrain generation. I am assuming you carved this out by hand, so this solution wouldn’t apply. Here is the thread I recalled, the solution being for smoothing terrain generated with a heightmap. Sorry to disappoint.
https://devforum.roblox.com/t/smoothing-out-terrain-heightmap/51242
Not by hand, I imported it using the new hight-map feature. I’ve just realised that its to do with the actual image itself, the download did warn there might be some bumps and that it said to use another version which I have now done. I’m just waiting for the new decal to be accepted then I can see it if it worked and I’ll update this accordingly.
EDIT: It works perfectly, it was a issue with the image itself. Thanks everyone for you help though 