Add your song, paste your words, tap along to set the timing, then export a video.
Export plays the whole song once in real time. Keep this tab open and visible until it finishes.
Press Start syncing, then tap Next line (or the space bar) the moment each line starts. When a part repeats, tap Previous to go back. Use Gap for music-only parts.
While syncing, you can also tap a line in this list to show it right away. When not syncing, tap a line to jump to it.