Pick a scene. Mute the actors. Be the voice.
If your voice lands late compared to the yellow real-voice bars, slide this left. If it lands early, slide it right.
Scene length: 0:00. Keep it under 2 minutes.
Each line is its own cut. Hit Record on a line: the video backs up a couple seconds, counts you in, and tells you exactly what to say. Lines you don't record keep the real voice.
Your recordings show up here. Record as many takes as you want and pick the best one.