Using the "Slide Show" tab select the "Record Slide Show" button
A dialog box will appear and you want both boxes checked. This will record your narration and will also allow you to use the virtual laser pointer while you talk. When you are ready, press "Start Recording". The recording will start on the currently selected slide. This will allow you to come back and correct your narration on some slides without redoing the whole thing.
While you are recording this timer box will appear. This tells you the elapsed time on this slide and on the whole presentation. This is very helpful in pacing your work.
You should have a good microphone that is close to your mouth so that you do not pick up lots of other noise and echoes.
To record, you simply talk into the microphone while stepping through your slides just as you would in a live presentation. Note that there is a pause button if you need to take a break part way through.
Your cursor will not show on the recording. If you want to point during a recording, hold down the control key and the left mouse button. A laser pointer symbol will appear where the mouse is. This will allow you to point at things while to talk.
When you are done, use the ESC key or the close box in the timer window to stop your recording.
When you stop, PowerPoint will pause for some time to save everything you recorded. This will almost always be longer than you expect.
When the recording is finally saved, PowerPoint will drop you into the view shown below. This is frequently not the view you expected. Go to the View tab and select the Normal view