In total darkness take the film from the cartridge or film holder. If it is in a commercial 35 mm cartridge, you will have to pry the end off the cartridge and cut the film free from the spool with scissors. For this experiment you need only pull the film from the holder and load the developing tank. You may want to practice with a scrap of film first, so that you can see what is happening before you have to feel your way through this in the dark. Once the film is in the tank, and the top is secure, you can turn on the lights.