first of all i would say not to stress about it! you can generally easily fix or remake something you might have done by mistake or end up not liking, you should definitely take a screenshot or picture of your map originally just in case you want to go back to it at some point or use it for reference and for peace of mind
it does help to have some kind of idea of what you want to do before you start, you could make some moodboards or try to imagine what it is you're trying to make or the feeling you want that area to have, do you want this area to be flat and easily accessible? do you want this area to be surrounded by cliffs and rarely seen by visitors? do you want a giant waterfall here, or a small pond there with a cool star or heart shape?
i try to keep the terraforming changes that i make simple, online you see things like "waterfall caves" and some crazy waterfalls designs, but i wouldn't worry about doing any of that stuff unless you like how it looks, in which case there's youtube tutorials i believe for some different setups so you shouldn't have to stress about how to make them! you can definitely play around with the tools yourself to see what works and what doesn't, without completely redesigning the whole island
i would also say don't try to do everything at once and change the whole map in one day, maybe work on 1-3 different areas each day, remember that you can always keep going back to adjust things and i find it a bit easier and certainly less overwhelming this way
tldr; keep it simple at first! take a screenshot of your original map, make some moodboards to get an idea of what you like
