Read the play for which you are designing costumes. Make special notes of the time period of the play as well as any occupation of characters that may require a specific type of costume. For example, a soldier will need to be decked out in a uniform, while a society woman will need to be dressed fancy. Often, playwrights will include specifics within the dialogue or stage directions that allude to the characters' costumes. Make special notes of these requirements as well when you begin planning the costumes. Make a complete list of characters, along with any special notes about costuming found within the context of the script.
Begin sketching the costumes. You will need to create one sketch per costume, so there may be multiple sketches for a single character if a costume change is required. Begin conceptualizing not only the individual character costumes, but an overall theme. Make sure that your designs will not clash with each other or the set design on stage. Costumes are meant to enhance the performance of a play, not detract from it by being too busy. Period pieces may require additional research into the clothing style of the time. Be as accurate as possible in the timeliness of the costuming.
Once the sketches are complete, you will need to procure fabric samples for each costume piece. For example, for a sketch of a man's work costume, you will need a swatch for the shirt, tie, and slacks. Attach these swatches to the actual sketch you created. If possible, procure pictures of specific hat or shoe styles required for a costume and attach these alongside your sketches as well.
Once all the sketches are complete with fabric swatches, lay the sketches side by side and view the overall coloring effect of the costume theme. Look for clashing colors as well as redundancies. While it is generally advisable to keep a single character within a uniformed color scheme throughout the play (it makes the character more easily recognizable to the audience), you don't want all the costumes to look identical. Clothes are as diverse in the theatrical world as they are in real life.
This is also a good opportunity to allow the actors to review their costume designs. They may be able to provide input as to the actions of their character in regard to movement that may require some consideration when costuming. Be sensitive to the input of your actors, but remember that, as the designer, the final decision on costuming is yours.