Your camera fills the screen, the script rolls over the top of it, and the phone records while you read. The words are never in the video.
First time: your phone will ask for camera and microphone. Say yes, or nothing records.
Filming: pick a script, tap the red button, three two one, then the words roll and recording starts together. Tap the button again to stop. Keep your eyes on the two gold marks at the top, that is right by the lens.
Afterwards: watch it back, then Save to keep it. On an iPhone that opens the share sheet, choose Save Video and it lands in Photos.
Mirror flips you like a mirror, and the saved video matches what you see. Turn it off if anything with writing on it is in shot. Speed and size stick between takes.