hi
This may help other users as well.
Given below is steps for version 10.1.4, using it on mac OS - Sierra.
Open imovie and bring the new background video or image to timeline.
Bring the video shot in green screen also on timeline above the first video.
make size of image or video similar by simple drag from corners
select the green screen video, click on video overlay settings and then choose green screen from background and we are done.
If you wish to have a look at video that has steps with directions on how to improve quality of edit of green screen, you may watch it, sharing link https://www.youtube.com/watch?v=-SmNkKHsR_I
If you still face any issue, please let me know