Examine the script. Take notes on the actions of each character and how the character lives. Identify any specific mentions of wardrobe in the script. Compare script notes to the director's notes to see if they are identical -- or if the director has a different vision of what his film will need.
Make sketches of the suit. Use your notes as guideposts to creating your sketches. If the character fights crime in a rough neighborhood, for example, he may need a cape that is frayed on the edges or tattered in spots. Give the director several different sketches as to what the suit could look like. Use colors, fabrics and cuts that enhance the feel of who the character is and how he feels about himself. Make the suit emblematic of the character's emotional and psychological state.
Create the suit. Take whichever image the director chooses and craft from that pattern and design. Obtain suit measurements from the actor playing the part. Cut and construct the suit to these dimensions. Use the movie budget to your advantage and purchase only what items are necessary -- Bummer caps for Union Army soldier costumes, for example. Take remaining monies and purchase the necessary fabric and materials to make one or more of the same suit. Having more than one suit on hand is critical in case the first suit rips, gets stained or is destroyed. Multiple suits prevent you from scrambling once filming starts and keeps the production moving in the event of a wardrobe malfunction or mishap.