The crux of creating such masks is that the mask edges are never clean and simple. Like the image in my demonstration video, the mask between the sky and the foreground can be complicated with lacy trees and branches. Same for interiors with plants and other complicated objects in front of the window. How do you create masks like this without spending hours in Photoshop?
As everything in Photoshop, here are many ways to do this but here I show you my favorite one since it is so fast and easy. It may look daunting at first but if you look carefully you'll see there are just three simple steps. The lacy trees along the skyline make this image harder than most. With this approach on many images that have mostly hard edges along the skyline you can just increase the contrast of the mask and quickly get a perfect mask. With this example you can't do that because of the delicateness of the trees along the skyline... believe me, I tried.
I'm still working on improving the quality of these screen capture videos. Phil Meadows and I are thinking about doing a whole series of post-processing videos. What subjects would you like to see in this area?