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Measure yourself in three places: while wearing shoes or boots, measure the distance from the nape or your neck to the floor; stretch your arms to the side and measure the distance between your finger tips; and measure the circumference of your neck at its base. Bring them with you when you go to the fabric store, and ask someone to help you determine how much fabric you need to purchase.  Allow yourself extra fabric, about 2.5-3 inches on two sides, for hemming. Fabric is usually sold by the yard. You can find patterns for a basic super hero cape online or at a fabric store. If you don't know how to use a sewing machine, there are a variety of cape designs available for different skill levels. Additionally, Darth Vader's cape is different from a standard super hero cape in that it doesn't "swoosh". You want to use heavy fabric to weigh the cape down. For a basic cape design, you will need:  At least 1 yard heavy black fabric (this is to make a child's costume, adults will need more based on height and weight) At least 1 yard additional fabric if you decide to include a lining. Pattern paper All-purpose black thread 2-3 inches of velcro Erasable fabric marking pen or chalk Sewing machine Using chalk or an erasable fabric marking pen, transfer the pattern from the paper onto the fabric. Adjust the pattern to accommodate your measurements (height from nape of neck to floor, and width of outstretched arms). Measure the width at the base of your neck to determine the collar size, adding several extra inches for comfort. When finished, cut-out the fabric.  Repeat this step if you want to add a lining to your cape. Alternatively, you can draw, cut-out, and then sew two semi-circles together. Fold the fabric in half and pin together. Double your neck-to-floor measurement and mark it on the fabric, leaving about 2 inches from the base of the semi-circle to the edge of the material. Use a piece of chalk attached to a string to draw a perfect arch. Cut the material out. In the center, draw and cut-out a smaller semi-circle for your neck. Use a sewing machine to stitch the two halves together. Use a sewing machine to hem the collar and bottom of the cape using a simple double-fold hem. Fold the fabric over about 1/2 inches, and then fold it again another 1/2 inches. Secure with straight pins. Sew the hem about 1/8 inch from the second folded edge. When done, flatten the hem with an iron.  When taking initial measurements for your cape, you want to add several extra inches for a hem. Hemming your cape will make it more durable and prevent the edges from fraying. Secure the collar together by sewing or gluing a 2-3 inch piece of velcro on each side. If your cape is heavy, you might need a slightly larger piece of velcro.
Take a few basic measurements. Select a pattern and purchase supplies. Draw the pattern onto the cape and then cut it out. Hem the collar and bottom of the cape and add velcro.