Select the pattern for the costume. You can find different styles of costume patterns at the McCall's or Butterick websites (see Resources). Measure the person who is to wear the costume before selecting the size and select the pattern from those measurements; the sizing of most patterns do not match the sizing of ready-made clothes.
Select the material for the costume. The choice of fabric will depend upon the use for the costume. If it is to be worn frequently, such as for a theatrical production, select a fabric that can be washed easily, such as a cotton or synthetic blend. Flat sheets can be used for costumes that require a lot of material, such as a Civil War or Renaissance dress.
Launder the fabric before sewing the costume. This will allow the fabric to shrink and prevent the costume from fitting improperly once it is sewn. Once it is dry, fold the fabric, with right sides together, and iron it.
Pin the pattern to the fabric. If the costume is to be for a theater, cut it a size larger; this will allow the costume to be worn by actors of different sizes. Use tailor's chalk to mark the darts and circles on the pattern. Cut out the pattern.
Follow the instructions for sewing the pattern. Use zippers, buttons and snaps if the costume is to be worn by one person. If the costume is for a theater, use elastic or hook-and-loop fasteners---like Velcro---instead of zippers or buttons to allow for easier costume changes.
Do the finishing work. No matter what the costume is to be used for, it's important that the hem and all trim work is sewn on securely. This can be done by using a sewing machine or, if you're sewing by hand, using multiple plies of thread.
Make a size/care tag for the costume. Use a scrap piece of material, about 3 inches square. Hem the fabric to finish the edge. Use a permanent ink marker to write the size of the costume and the fabric content on the tag. Sew the size/care tag inside in an inconspicuous location.