Advice for preparing a virtual presentation
First of all, you need to know the specific requirements from the conference host. Do they have a specific file format that they want you to use? Otherwise, you might want to do a screencast recording. There are free software available such OBS (Open Broadcaster Software) where you can record your screen and microphone at the same time. That way, you can go through your powerpoint presentation while recording both the screen and audio. Make sure you use a good microphone when recording and try to minimize as much background noise as possible.

Good luck with your presentation, I'm sure you'll nail it! 

PS: Let me know if you're struggling with getting OBS to work, it's fairly easy and straightforward so there shouldn't be any issues (also many guides available on youtube).