Recording voiceover audio involves adding an audio track to an existing video. In most apps, this is pretty simple. For example, this is how you do it in PowerDirector:
Select the audio icon and then hit the Voice-Over button.
Choose the point in the video where you want to record your voice and cue it up by moving the selector to that section.
Press the microphone icon in the middle of the screen, press the red Record button, and choose which audio track you want to use.
Record your voice, and if you make a mistake, just hit Retake to give it another shot.