When the shutter button is pressed, the shutter opens to allow light to enter the camera through the lens.
Light is focused through the lens onto the mirror at the back of the camera.
The light bounces off the mirror and down onto the film. The film contains layers of light-sensitive chemistry. As light hits these layers, the image is exposed.
The instant photo is ejected through the rollers. These rollers break developer paste pods in the base of the photo frame. A blue paste then coats the image, triggering a chemical chain reaction that both develops the instant photo and shields it from light.