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Assuming the vantage point of the separate image is the same as for the main image, add a similar shadow (possibly vignetted) to the one appearing at the feet shown in the original image. In addition, use the Gaussian Blur tool, if necessary, to match the lack of sharpness that may be apparent in the main image. If the vantage point of the image to be added is different from the larger image, almost anything you do will look faked.
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Adding to what norman said, you can employ some trickery when required. e.g., If the person's feet is "standing" on grass, you can select and copy some of the grass (from the original background image) and paste it (in the exact same position) onto another layer on top of the layer stack to make it look like the person is standing in the grass. Also, think perspective, lighting, shadows, color...
Exercise these things a lot and soon it'll become second nature. Compositors don't just jump into a composite; they'll usually study what they want, do some mock-ups and especially test out shots. Obviously, you don't want to put a subject into a composite that has a five o'clock shadow on their body into a background that shows a cloudy, sunless day.
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All good advice above from Norman and Warunicorn
Do you have an example of two images that you want to combine and we can try and show you how?