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 Sylvie Proulx.
You can use AKVIS SmartMask to quickly and easily cut even complex objects from a photo and combine them into a stunning collage. In this tutorial the author creates a magic portal, through which at any time wild animals and birds can enter your home.
These are the images which will be used for the collage:
Result:

Make a copy of the layer with the command Layers -> Duplicate, then open the AKVIS SmartMask plugin (Effects -> AKVIS -> SmartMask).
 Draw a line with the Drop Area Pencil tool 
 within the part of the image to be deleted (in this case the sky around the bird).
	
 Draw a line with the Keep Area Pencil 
 within the fragment of the image to be kept (the bird's silhouette).
	

 to process the image. The result will be shown on a transparent background in the Processing tab. 
	
 to apply the result. The image will be sent to the graphics editor.
	
 to process the image. The background will be deleted.
	If the edges look too rough, smooth them by increasing the value of the Mask Smoothing parameter and reprocess the image.

 to close the plugin's window and send the image to the graphics editor.
	
Press 
 to process the image.
	
. The image will be sent to the graphics editor.
	
- Choose the image of the frame;
- Select the layer with the frame on a transparent background (Ctrl+A);
- Copy the selection to the clipboard (Ctrl+C);
- Switch to the background image;
- Paste the selected fragment onto the background (Ctrl+V).
Transfer the processed images of the bird and deer in the same way. As a result, all four images are in one file, and the layers palette looks like this:

