It's deceivingly simple. I filmed myself holding a sheet of green construction paper. 

I keyed out the green in WeVideo, thus creating a transparent "window" that revealed whatever I layered underneath. 

For that, I chose a photo of a skeleton, which I resized and adjusted until it lined up to me as much as possible. Ta da! 

The top layer, Video 2, is the same as what you see here. I selected the clip and then used the color keying options in WeVideo to remove the color green. 

Video 1 (the bottom/background layer)= a skeleton found via stock content image search