It is important to check that your microphone is working before trying to record your voice over. Tips and tricks on how to do this are shown in a later tutorial
Review your animation and decide where you want to add your vocal voiceover.
Record your voiceover and review it in the voiceover library.
Select the frame where you want to the voice to begin and add it to the timeline by clicking add or "+" button.
Click on playback and listen to the voiceover. You can adjust the volume if you wish.