If you're 100% sure that you want to do your video simply to show you going through a conventional PowerPoint presentation ... same as you've been doing at conferences for years ... then all you need to do is learn (if you don't already know) how to use the PowerPoint tools that allow you to set the timings for automatically advancing to the next slide, and for recording your voice as you talk to your screen. You can also record your face if you want, using your webcam.

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These tools are all under the "Slide Show" tab at the top of the PowerPoint screen. There's a "Rehearse Timings" tool that simply records when you advanced to the next slide. With this, you can rehearse your talk, check to see how close to 20 minutes you are, fine tune it and get the narration and the sequence perfect. If you check the "Use Timings" box then your last set of rehearsed timings will be used to automatically advance the slides. If you save the PowerPoint file with that box checked, then when someone opens your PowerPoint file and starts the slide show it will advance at exactly the times you saved when you last practised the talk with the Rehearse Timings tool.

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One good way to get everything exactly the way you want it is to write out exactly the words you want to say and read them at your computer as you advance the slides. If you get to the end and you're at 20 minutes, then you're ready to record. If you're a bit over then you have to take a look at your script and decide where you can chop out some words, or maybe chop out an entire slide. Once you've got a set of slides and a script you can work through in 20 minutes, you can click on Rehearse Timings to go through the entire talk, reading  your script at a comfortable speed. You can then click the box that says "Use Timings" and on your next run through, which will be the one when you record your voice, the slides will advance automatically.

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With the slides set to advance automatically, you can click on "Record Slide Show" to get your voice narration into the PowerPoint file. When you're recording, you have access to tools for highlighting, and for using the mouse as a laser pointer. It usually takes a few tries to get everything right: your voice, the highlighting and pointing, etc. Once you've got a version that you're happy with, you can upload the file to the conference's Google Drive folder. From there, we can run your presentation on auto-pilot and create the MP4 video file.

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Here's one (of many) YouTube videos that explains how to rehearse the timings, make them automatic, and add voice narration to your presentation: https://www.youtube.com/watch?v=cllW-ybFQDg.