Click "Tools" and "Narrate Timeline"
The Audio screen will open. You'll see the "Start Narration" button in the upper right, with the "Stop Narration" button below it. To the right is the volume control, "Input."
About 2/3 of the way down the screen you'll see where you can click an arrow to choose your Audio device from a drop down box. I recommend using an external microphone because the sound quality will be so much better. You plug it into the USB port and your computer detects it and adds it to the audio device choices.
If you are not used to ad libbing you might want to write notes or outline what you plan to say in the narrative. You don't need to narrate the whole video in one long audio clip.
You can start and stop and make many smaller narrative clips. That way if you want to remove some of them and do them over later it's easier.
Wondering what to say? If it's a "how to" video you can give the sequence of steps. You could also report on what is happening at an event - who, what, when, where, why.
