First, you have to be in the desktop version of PowerPoint to record narration (just in case you're working in the cloud).
Then, look under Slide Show options, and there you'll find "Record Slide Show." Clicking that will launch your presentation, and you should see a red record button in the upper-left -- click it when you're ready to start talking.

After you’ve recorded the narration, you should then see a speaker icon appear on your slides that, when clicked, will give you a mini-control panel to play the recorded narration.

When you're satisfied with how everything sounds, just save and submit your presentation according to your instructor's guidelines.