What are the step to making a green screen

I know FCP, and I know how to take my video of me playing the guitar and putting an image or video behind it. I also know how to get some free Youtube green screen videos that I can put in, [ Crow fly from the back to the front and out ], Motion 5 has something like this. I've tried to import into Motion a .mov, png, mp4, and jpg, all green screen chroma, [ I use the iMove green screen image in FCP ].

So what are the steps to making a green screen [ Image or animation ] in Motion 5, and exporting out to FCP so I can make my own.