Measure your wrist, take 1/2" off the measurement and cut three strips of duct tape to that final size. Carefully adhere two strips to each other, sticky sides together, keeping the edges nice and even. Cut a third strip in half lengthwise and fold/adhere each half evenly over each side of the cuff to "seal" the long edges. Cut two more 1" long strips to fold over and seal the short edges of the cuff.
Cut a strip of cloth gaffer tape to fit within the length of your cuff and adhere it down the center of the outside of your cuff. Use the point of a pin to mark where you want to put studs or brads along both sides of the gaffer tape (the studs can be positioned to help keep the gaffer tape from peeling up over time).
Use a 1/16" hole paper punch to punch larger holes if needed to make it easier to push the prongs of the studs or brads through to the back side of the cuff to secure. Punch a hole in the center of each end of the cuff and insert a large heavy-gauge jump ring. Add a closure finding and you're done! I like the magnetic closures, they make it so much easier to put your cuff on to wear.