I'm a noob, so explain like I'm 5 years old.
- Put the video or image you want to use as the background (what will replace the green) on Track 1.
- Put the video that has the green screen on Track 2.
- Double click on the clip with the green screen background to open in the PiP Designer.
- Select the Chroma Key tab and then Enable chroma key
- Click on the eye dropper button on the Chroma Key tab and then select the (green) color in the image or video clip that you want to replace/make transparent.
- Adjust the sliders until the green completely disappears.
This video might help explain it better.