(Patrick Fitzgerald, Beargrass Media Director, giving a discussion on green screen magic at this past weekends' community-led workshop as part of the Louisville Young Filmmakers Festival)
Using Chroma Key
Using a green screen is a fairly simple process that can be done with a green cloth or brightly painted wall.
First, record a person or subject in front of the green screen and add it your video editing software. Next, you will need to utilize a tool called chroma key to remove the green leaving just the subject. (Tip: don't wear clothing that is the same color as the green screen, or else the background will bleed through.)
Next, add another background image or video clip in the same time frame on the storyboard of your video editing software to place the actor in a new background. (If you zoom in on your actor, be sure to zoom in on the background to match.) Adjust your sound levels and export your clip.
Free Video Editing Software
Here are some freely-downloadable software programs that have a chroma keyer:
For more information, be sure to check out some of the many video tutorials available on YouTube.