When you're finished with your drawing, press the checkmark at the bottom right of the screen. The app will take a final picture and also generate a time lapse video of the drawing process.
Both your image and the time lapse video will be saved to your camera roll. You will also be given the option to share the image and the time lapse via email.
It's worth noting that Masterpiece will only generate a time lapse video when you've drawn for at least a couple minutes, otherwise, it won't have enough data to actually make a video.