Photoshop help needed (merging pictures)

[TT]

It should be simple, I keep seeing it in every kind of works but I can't really understand how to do it proprely... yes, you can call me noob!

Do not laugh, it is about dolls LOL. My girlfriend would like to be able to have something like this other artist does, for her own dolls.



You see, it is just about merging some pictures together, but how can you do it so well?

You know, to do it so that you don't see where a picture stops and the next starts. I can't even understand which background belong to which picture in this exemples.
You basically have to take the pics thinking about the future merging, ence paying attention to the background color or it's all about doing the proper merging and avoiding sharp edges
In the third one for instance, the basic background seem "articficial" and not the real one, but how to cut the pics and paste them on it that well?

I hope you understand what I mean and I hope somebody has some quick tips to do it

thanks in advance!