What I found important was that I also focussed on the area’s where the real world can overlap with the painted world. I managed to do this by copying (duplicating) the layer of the real photo and put the layer with the painting in the middle. By doing so you can achieve some form of depth in the image.
The photo with the apple tree and the clouds are good examples of this. It does require a good deal of masking and that can be a lot of work.
There are many different options and possible setups, so one thing I know for sure, we are not even close to the end of trying this technique. Junior and me are already planning the next shoots.