Summarize:

Depending on the kind of camera you’re using, you may need to rewind the film into the cassette using the handle on the side of the camera. Don’t open the film once you take it out or the light will ruin all of your pictures. Don’t worry about turning the lights off yet. Since you’ll be working in complete darkness, you want to set up beforehand so everything you need is right in front of you. To load and develop your film negatives, you will need:  A film reel. The reel is what you’ll load the film onto once you take it out of the cassette. A film tank. A film tank is a sealable plastic container that you’ll develop the film negatives in. A cassette opener. You’ll use the cassette opener to open the film so you can load it on the reel. Scissors. You’ll need scissors to cut the film off the cassette. At this point, there should be no light in the darkroom. If you can see in front of you, there’s too much light. To open the cassette, hook the edge of the lid under the cassette opener. Then, bend the cassette to the side until the lid pops off. Make sure your phone is turned off and put away so it doesn't light up and ruin the film. Unroll the film until you reach the small plastic cassette in the center. Then, cut through the film where it meets the piece of tape that attaches the film to the plastic. Since it’s dark, you’ll need to feel where the tape is with your fingers. To load the film, start by holding the reel in one hand and the end of the film in the other. Then, find the slit on the edge of the reel with your fingers and slide the film into it. Once the end of the film is secure on the reel, twist the side of the reel back and forth to wind the rest of the film onto it. When you’re finished, all of the film should be securely wrapped around the reel. There shouldn’t be any film sticking out. First, slide the tank’s detached core through the hole in the center of the reel. Then, place the reel flat in the bottom of the tank so the core is sticking up in the center. Cover the tank with the lid and tighten it into place by rotating it.
Take the roll of film you want to develop out of your camera. Bring your film into the darkroom and set up your work area. Turn off the lights and open the film cassette with the opener. Take the film out of the cassette and cut it off with scissors. Load the film onto the reel. Place the reel in the film tank.