One thing I notice among a lot of inexperienced manipulators, is using over dramatic color correction to cover up mistakes and 'blend' elements together.
You should be practicing getting one element into a photo and match that element to the existing photo.
Once you can do that very well in different areas (sky, ground, sea, hills, flat ground, bricks, grass, ect), then step up to more elements still while not adjusting the base stock.
Then when you can blend to match a stock, go crazy make some atmospheric color corrections and crazy depths.
Hone in your skill before trying to step too far ahead of yourself.