Place the footage you want to be the background of your final composition into your iMovie project Timeline.
Select the portion of your green screen footage from your Event and drag and drop it on top of the background footage in your project. You must be precise in dropping the green screen footage right on the background footage.
A contextual menu will pop up. Choose the Green Screen option
iMovie will composite your footage together. You can adjust the placement of your green screen footage by sliding it to the left or right in the project window.