Introduction: Fish-eye Lens Effect for Phone Camera
1. Unscrew the "business end" from a low-cost door security viewing lens.
2. Fix it (temporarily) over the lens in your phone camera. Luckily mine was a snug push-fit into the rubber case, but Blu-Tack® or similar would have done the job just as well.
3. Have fun making pictures.
I like the blurred reflections from the shiny brass tube, but that's purely a matter of personal taste, so if these bother you, simply cover the inside with a roll of thin, matt-black paper.