Hi there,

Just place the play cursor where you want the voice-over to start. Then hit the red, circular recording button under the source monitor. Pick "audio" in the window of recording options that will come up. Check and adjust the settings for your microphone recording and hit the red recording button in the window to start the actual recording process. That's it. Once you're done, hit the stop button and choose to keep the take. Another audio object will be created in the next track and you will still hear the original audio from your imported video. Volume levels can still be manipulated after recording for an ideal mix.

I hope this helps to get you started.