This site has many fun resources to keep your students engaged!
You can use a green sheet, paper or anything you can find that is green. I ordered mine from Amazon. Regarding the video background part, you would not have to make a JPG file of your message, just do the same steps as in the video and select your uploaded video file instead of a picture file. Then the video will show in the green screen area only.
Let me know if you have any questions! Your favorite green sweater is out for a while unless you would like to become part of the background! ;)