Combining multiple images is something that every photography will try at some point, but it is more difficult than merely cutting out one part of a photo and replacing it with another. This video teaches several important skills for matching the various images of a composite.
Creating a composite is more difficult than you would think. Producing a proper cutout or mask is often hard enough. After that is where you have your work cut out for you. Matching the lighting, colors, and producing the proper level of blur in the background are all steps that take multiple skills and a good eye. In this tutorial, Nemanja Sekulic gives a complete explanation on how to swap out a background and then adjust the layers to make the final image look realistic.
Sekulic uses several techniques to produce his final product, including the pen tool, selective color, curves layers, and smart objects. For a demonstration of these alone, the video is worth watching. However, the main thing I learned was the importance of blurring the different parts of a photo to remove the fake look that a composite can often have. This step is the one that is most often forgotten and is one of the most common reasons for a failed composite. Sekulic not only applied a Gaussian blur to the new background in the image, but he also used the blur tool on several areas that made the image's transitions look more natural.
If replacing a background or creating a composite is something that you are currently working on, this is an excellent tutorial to watch.
Gaussian Blur? Gag! Try lens blur.
I suppose you could use the blur tool on the layer mask if you didn't want to commit it to the image.
Nah, cause then you're just fading in the part of the image you're trying to cut out. You'll end up with a fringe of the over exposed exterior around the window sill.