To edit green screen in iMovie, start with an empty project and select the background you would like to use. iMovie has some built in backgrounds, or you can use an image that you have produced or an image that you have imported.
Now, drag and drop the background image (or video) into the timeline. Ensure that you drag the background onto the timeline before dropping the footage with the greenscreen.
Select the green screen footage you wish to use and drag and drop it above the background. If your green screen footage is still on your USB drive, you will need to import it into iMovie first. Import media by dragging and dropping from the desktop, using the libraries on the right-hand side, or by going to File > Import.
To make your background image shorter or longer to match your green screen footage, drag the edge of the background to match your footage. Similarly, to cut footage from the green screen footage (such as the first few seconds of you pressing the button), drag the edge inwards.
Double click to select the green screen footage in the timeline, and then select the drop down menu that is labeled "Cutaway" above the left corner of the viewing window. Select "Green/Blue Screen."
The green screen should now be removed to reveal the background behind it!