You need:

• microphone
• installed PowerPoint, not cloud-based PowerPoint
• recommended - if you add Notes to your slides, you'll be able to reference them while recording

Instructions:

In PowerPoint, go to Slide Show > Record Slide Show > ...

• ...Record from Beginning, if you want to do everything in one go.
• ...Record from Current Slide, if you want to go one side at a time, or re-record a slide.. (Of course, you should already be on the slide you want to record for!)

Recording View:

• Record by hitting the red Record button in the upper left. You can stop and pause, as well as play back your recording.
• Display your slide notes with the button in top-center to stay on target.
• Keep track of how long you've been recording by looking at the bottom-left.
• Turn your mic and/or webcam on or off in the bottom-right.
• If you need to highlight something for emphasis on a slide, there are pen and highlighter tools (plus color options) at bottom-center.

Finish Recording

• Press Stop when you're finished talking. (Use pause if you need to get your bearings or remember your train of thought.)
• Hit the Esc key to exit the presentation/recording view.
• A speaker icon will automatically be added to the bottom-right corner of your slide(s). Hovering over it will reveal playback controls.

• Save your PowerPoint like normal to submit (or otherwise follow whatever instructions you've been given).

You can test the overall presentation by launching the normal Slideshow presentation mode -- your narration should automatically play for each slide.