If you ever do any sort of composite work, chances are that you'll need to add shadows. They're one of the biggest aspects of making a composite image convincing, and yet, they're also very subtle and tricky to pull off. Phlearn is here with a great tutorial to get you started.
I'm sure you've seen bad composites in which the shadows just aren't right or they're nonexistent. It makes a person look like a cardboard cutout propped up on the ground, or worse, it makes them look as if they're floating. Part of the trick is understanding that not all shadows are the same, and thus, there is no one size fits all way of creating them. A lot of creating a convincing effect relies on being able to read the light in an image: there's a correspondence between the type of light used and the type of shadows cast. Be sure to notice how Aaron Nace samples the color from from other shadows of the image instead of using straight black and how he uses multiple layers and gradients to build up the effect. Using a single gradient and layer simply won't work in this case. Check out Phlearn's YouTube page for more!