The example was made in the old version of SmartMask. Now, you can achieve this effect more easily with the new AI-based tools.
The author of this tutorial is Alvaro Salazar Chacon. He demonstrates how to use the SmartMask plug-in to change the background of an image (the following example the sky) regardless of the complexity of objects in the landscape.
	




 Click on 
 to start processing the image again.
	
The fragment was selected well, but there are still complex areas from the original background.

 button.
	
The result is much better but there are still areas that I want to remove.

 Use Zoom 
 to enlarge problem areas.
	

 Activate the Magic Brush tool 
 and use the eyedropper tools 
 to pick up the colors for the Keep Colors and Drop Colors lists.
	
Then select the Magic Brush tool and use it on the image.

To improve the result further, click on the Foreground Recovery check-box and choose the color black:
 
 Re-treat areas with the Magic Brush 
. The fragment looks perfect.
	

 button and the image will be returned to the graphics editor.
	Now you can create landscapes with different backgrounds: