There's a little bit more to it than that.
Choosing source images carefully so that shadows and lighting are reasonably close is important. Choosing colours, and recolouring things that don't match, is one of the trickier things. I am 100% self-taught as far as this kind of thing goes, so I don't know how the pros do it. I just do it my own way.
If you have a specific project in mind, post your original here on the forum and describe what you want to do with them. Maybe I and others can give you some tips on how to go about it.
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