If you are asking about how to add a narration track to a completed project, open your project, go to where you want to start recording your voice, and on the monitor, press the red record button.  A dialog box will pop up asking what source you want to record from.  Choose audio, another dialog box pops up where you can choose how you want to handle the audio such as internal mic, external mic etc, what file type you want the audio saved as, and where you want to save the audio.  When you are ready, press the red record button in the bottom right hand corner of the dialog box and start your narration.  when you are done, press the stop button to the left of the record button, and your narration should show up on the next available track.  If you have a soundtrack with music etc, you are probably going to need to mix the audio to get the blend that you want.  I hope this helps and good luck