Get the green screen footage. It is important when setting up your green screen that you get the fabric as wrinkle-free as possible. The most effective technique is to use a steamer. You can also use clothes pins to stretch the fabric. Also, make sure no part of the subject is green, including clothing and props. If there's a similar color in the foreground, it might display the background image instead of the subject. There should be some separation between the subject and the green screen because you will need to light them separately. Make sure the light is evenly distributed across the green screen.
Import the green screen footage into the editing program. To do this, plug the camera into your computer. A firewire cable is the fastest way to import footage. There will be an import menu option somewhere in the editing program, depending on which program you use.
Get rid of the green or blue. This process will vary slightly from program to program, but it essentially involves selecting the color of the green screen and selecting a menu option.
Place the footage over your existing footage. The subject will not appear unless it is in the foreground. Some programs will just require you to put the green screen footage above the existing footage in the timeline. Some will ask you to combine the two.
Fix any problems in post production. Make sure you consider the perspective of the subject compared with the image that will be displayed in the background. Otherwise, the subject might look too big or too small to match the background. If the subject is lit brightly, but the background is at nighttime, it won't look good. You might be able to fix this with color correction.