Research clothing that is similar in style and time period to the costume that you are designing. This research can be done online, or in costume and photography books.
Visit a fabric store to view fabrics. Most stores will give you small swatches of fabrics that you are interested in. Collect several possibilities for each article of clothing.
Download a body template from the internet and print several copies. These templates are simple line drawings, intended to be used as a basis for your design. There are links to a few templates in the Resources section.
Sketch the outline of your costume using a mechanical pencil. Experiment with several variations on different sheets of paper. Erase as needed if you are unhappy with your design.
Fill in the drawn clothing with color and texture using colored pencils. Use the fabric swatches that you have collected as a guide. Try different fabric designs to find appealing combinations.
Add additional realism to your drawing by drawing natural highlights and shadows into the design. Decide where the light is coming from and shadow accordingly.