Terraforming is something that takes a lot of time to learn. It requires many, many days of practice, and its not something you should rush. I can give you a few tips though. When you make curves, avoid placing blocks in a uniform matter, you want to mix up how long curves are. You should also mix up the blocks used, like to choose blocks that go well together and add small amounts of then around the main block. If you want to make larger things fast, I would recommend using the //br command in world edit to make a decent shape, then manually make it looks natural.