Obtain 35mm film. This could be undeveloped, developed with nothing on it, or have pictures on it! The film on my bracelet is from the end of vintage movie film I got from a theatre I worked at.
If you are making this for someone who liks photography and don't have film yourself, you could buy a disposable camera and take some pictures with it just for this.
Using some sort of felt, like my cuff, or other strong fabric, sew the film onto a strip just wide enough to fit it. Use the sprocket holes as guides to sew.
Leave space at the ends with no film covering the cloth to sew on a snap button or clasp of your choice, and you're done!
If your film smells funky, kind of like vinegar, hang up your film, making sure the entire roll is unraveled, on a bulletin board or your fridge a few days prior to making the bracelet and the smell will go away.