Hi and thanks! If you'd like some pointers on how I created the landscape I did a couple of explanations of my thought process - with pictures - on my instagram (search for bena_lego), or there are lots of pictures on my Flikr that show work in progress (link is in the original post, but be warned my Flickr is very disorganised!).
Other than that, my advice is:
Gather plenty of reference material and use that for inspiration (I had lots of photos of real world streams and waterfalls, and I googled the artwork from the elves series to get a feel for the colours etc). You don't have to stick to them rigidly but I found having reference material to look at really helped me decide what I was going for, and also keep me consistent when my build took a long time.
I sketch what I have in mind before I start building and that helps me spot and work out major problems early on. It also gets me thinking about how I might build it. Sketching things out also lets me plan how ambitious I can be. At the beginning I underestimated how many bricks it takes to build even just a small piece of detailed landscape and it was quite disheartening to have to stop and wait when I ran out. I learnt that sketching it out first let me find the right balance between what looked good and what I could realistically achieve - and also let me plan bricklink orders in advance!
Once building I always go for interesting angles wherever I can, from whole chunks of scenery right down to individual rocks. I think most serious Lego moccers would agree that building 'off grid' is a very good way to make things look natural. I also try to make sure there's always a logical way for a minifig/doll to reach each part of the landscape, so lots of pathways, ladders, stairs etc... but that's just a personal preference and I don't always stick to it!
Don't be afraid to change your mind or start over. I lost count of the number of times I rebuilt something on that moc because it didn't look right! My version of Rosalyn's blue Healing Hideout was especially tricky - must have rebuilt that 6 or 7 times. I also sometimes take a photo of the build in progress, then draw over the top of it to give me an idea of what to do next.
Finally, enjoy yourself and don't forget to post whatever you build here - would love to see it!