Choose which Batman you want to copy. The Dark Knight has worn many colors and costumes. He has his traditional gray and blue comic book uniform, his black and gray costume and several variations of black leather suits used in the more recent movies. Choosing Batman's suit will determine where you will start.
Visit thrift stores to look for tights and tight-fitting athletic clothing that would be suitable to turn you into the Dark Knight.
Go to the fabric store to find fabric for the cape and the cowl. Pleather, or imitation leather, is a good choice for the movie version of the Dark Knight. Some stores may carry sewing patterns to make a Batman costume or a superhero costume that you can adapt. Use small styrofoam cones for the ears and cotton for padding for musculature in the chest and abdomen.
Assemble the suit. Sew the cape and cowl and add them to the thrift-store clothes. Refer to Batman comic books or sewing patterns as your guide. Cut out a bat emblem from fabric by copying pictures in the comic books and sew it or use a glue gun to adhere it to the chest of your costume.
Create the utility belt. Purchase a toolbelt and use spray paint to turn it bright yellow or golden to match the version of Batman you chose to emulate.