First you just need to insert your background image or video and add a green screen video footage you shot, or you can just insert one copyright free footage from YouTube (just make sure you have the rights to use that footage). To learn how to insert video on your PowerPoint, watch this tutorial
Then edit the Alt Text of the green screen footage and insert %%GREENSCREEN% text in it. If you want to mute the Youtube video, add %%MUTE% in the Alt text.
If your footage is using Blue screen instead of Green Screen, change the text to %%BLUESCREEN%
Now the sky's the limit ! You already have the tool to create professional looking videos !