Before the actual design begins, the costume designer works with the director to develop the overall look of the film and the personality of each character. He reads the script thoroughly and understands the tone of the piece, as well as how the costumes will fit into the visual palate. He speaks with the set designer, lighting director, make-up director about how his work meshes with theirs. Once actors are cast, the costume designer works with them to get their particular take on the characters. If the movie is a period piece, he needs to do research into the historical time, understanding both fashions of the period and the types of fabrics which were used. With that as a basis, the process of costume designing can begin.
Now fully immersed in the particulars of the production, the costume designer produces sketches of each character's outfit. Each character can merit dozens of different sketches, varying the look and fabric to give the director as many options as possible. A gradual process of refinement takes place as the costume designer gets closer and closer to the desired look. Eventually, she settles on one specific image for each outfit involved in the film and uses that as the basis for the actual design. The sketching step may overlap with the initial information-gathering step, as the designer makes preliminary sketches while performing her research.
With the basic costume design in place, the designer then meets with his staff to talk about implementing it. This can include cutters, tailors and wardrobe managers: anyone involved with putting the costumes together. Their discussions center around the type of fabrics they should use, the specific patterns of the costumes, and the ways of meeting the film's costume needs on the budget provided. They then purchase what they need -- raw fabrics, zippers and buttons, or even just premade clothes off the rack -- and assemble the costumes. During this period, actors come in for fittings to ensure the costumes look their best, and the costume designer may artificially "age" the costumes to make them look worn if the script calls for it.
Once the costumes are done, the designer often remains on set during rehearsals and production. Little accidents happen all the time on a movie set, resulting in rips or tears to the outfits. The costume designer, or her assistants, can be on hand to fix any problems or make changes to the costumes very quickly, ensuring that the production remains on schedule.