No sound on iPhone? Turn the volume up and make sure the page has been tapped once. If you hit late on every note, move the offset up. Bluetooth headphones usually need +100 to +200 ms.
Beat grid
The game listens to the song and works out its tempo. If the notes feel off-grid, type the right BPM and re-chart.
Audio
Your own chart
Record mode plays the song and saves every tap as a note, snapped to the beat. Hold a lane to record a long note. Keys: D F J K, or tap the lanes.