Compositing images well is one of those skills that can really flex your photographic prowess by calling on a multitude of techniques, all of which you have to have a sound basis in to create a convincing final product. This great tutorial will show you all the steps.
Successful compositing takes some forethought and planning. In this video, Nathaniel Dodson walks us through this by first adjusting the background image, then showing how to cut out the model. In this case, it provides some good practice in cleaning up selections, as the model was shot in front a brick wall, and the rim light is fairly close to the mortar in color, as is the skin tone to the brick itself. Next comes the process of matching the two images: color and tone work, dodging and burning to match lighting, then adding lights to the stadium to complement the overall theme of the portrait. Several more rounds of color, lighting, and sharpening work follow.
At 45 minutes, this is definitely a longer tutorial, but with that length comes a comprehensive understanding of the process and the tools involved, and I highly recommend taking the time to sit down with it. Share your creations in the comments!