Think of a topic that you think it is interesting and research it. Take notes.
Think of the essential elements in game design. Analise some of the games below to get some more ideas. Start designing your game: story, aim, rewards, etc.
Do not forget to read the rubric to know what a perfect game will look like.
Remember to prepare a storyline and a script.
Have everything ready so that you can explain the rules in a clear way. Explain how to start, how to end, and who wins very clearly. Maximum 3 minutes.
Use the video on your cell phone and then edit it in a video editor (such as Movie Maker or iMovie). Alternatively, you can use https://screencast-o-matic.com or https://explaineverything.com/ to capture any area of your screen, allowing you to add narration from your microphone and video from your webcam.