Measure everyone who will be performing in the play. This includes the bust (or chest for women), waist, hips, length from waist to knees, length from waist to instep, around upper arm, around wrists, shoulder to shoulder and shoulder to wrist. Record these measurements on individual charts for each actor.
Talk to the director of the play and learn her "concept" and plans for the show. Directors have their own style and ideas, and it is very important to include her at this stage of the planning.
Research the time period of the play in which the costumes designs will be used. Look up pictures, colors and patterns.
Determine your color palate. Color plays an important part in the presentation of the play and in the concept of the director. Keep the director in the loop at this stage of the planning. Keep in mind the skin and hair colors of your actors when planning the colors and use them to enhance, not detract the character he is playing.
Present the director with a set of material swatches representing the color scheme of the costumes. If you are artistic, sketch examples of the main character's planned costumes.
Hire or recruit several seamstresses to help with the sewing if you are creating costumes from scratch. Cut out patterns using your selected material. If you are recycling clothing or old costumes, it is still wise to have helpers to make alterations to clothing used.
Do preliminary fittings with actors, and keep a record on their individual charts of what has been done, which costume pieces are finished, what is lacking and what needs to be changed.
Finish changes and final touches.
Arrange with the director a time for a 'costume parade' on the theater's stage to give him a chance to view the costumes as a whole before the play opens.
Lay out your costume policies and rules to the actors. Let them know what they are and are not allowed to do in their costumes, such as smoke, eat or drink. Also make it clear that they must hang up their costumes and take good care of them.
Enjoy the show! Be aware that depending on the length of the run of the show, you may have to be available at the theater to make repairs to damaged costumes and wash some as well.