Download the Pre-Keyed Electric Effect: http://specialeffects.imoviehowto.com/how-to-do-the-electricity-effect-in-imovie-11/

First, you need to google image search “chroma key green”. Then you need to select a picture from the list and download it. After you put the the chroma key color picture in your iMovie project, drag your green screen clip on top of it. Select the picture in picture option instead of green screen when the menu pops up. Once you shrink yourself down to the size you want, export the project by going to “Share”, “Export Movie”. Once you exported your project import the project back into iMovie and take the part you just worked on and drag to the background you want to apply it too. Now select the “Green Screen Option”.