First you need an application that can handle selective colour selection such as Photoshop. ( Might be able to do it in Elements but never tried it.) Next you need a green screen, pretty obvious really, but you need one that is a genuine "Green Screen" rather than a green cloth. Next is the tricky bit. you need to ensure the screen is smooth and importantly is evenly illuminated. This is trickier than it may seem especially if you have a large group. Final step is you place your subject in front of the green screen, ensuring they are not to close and watch out for shadows. Ideally all this is done in a fairly large room or studio. Finally you use the selective colour option within your editing app to select the"Green" area make it a transparent area in the image. You then drop this image onto the background of choice. You may need to a bit of cleaning up round the edges but that's what you do in a nutshell. Sounds simple, and sometimes it is. I'd Google "green screen techniques" to get a more detailed description of the technique
