Click on the Mute video button to use the tool. A picker will be shown to select the background video.
In this screen, you can add overlay videos, use the control points to change the size and position of overlay windows. You can also select the starting time of each overlay video at the bottom slider.
Click on the Save video button to start the process. You can choose the output format (MP4 or WMV) and quality in the dialog. Then click on the Make movie button.
An other picker will be shown to select where to save the ouput video.
After the processing is complete, you can preview the output result.