Robb. Knight.

Green screen, if you don't know, is a technique used to place footage of something over the top of another image or video. Many think that this sort of feature is limited to advanced movie editing software such as Final Cut Pro but this just isn't the case. Green screen is not on by defualt in iMovie so the first thing you need to do is turn it on. Open up preferences (⌘+,) and tick the 'Show Advanced Tools' checkbox.

Now that these are turn on you can start to overlay your green screened movie on top of images or other videos. To start with, you need to drag the _background _image/video into iMovie, either from the photos/video tab or from finder.

Now drag your movie with the green screen over the top of the inserted image, and you will be shown some advanced options.

As you can see, not only can you use green screen, but there are a multitude of other options that were not there before. Select green screen and check out how your movie looks. Depending on the quality of your original video, you may need to adjust the colour settings to get the right look.

And that's it. It really is that simple. If you want to change the background image/video halfway through the video, simply drag the new background next to the first one, and adjust your video length. Here, for anyone who is interested, is the finished video shown in this tutorial: