1) This is the paper pattern we will use. The pattern is an "X" with the upper left arm extended. Draw out the pattern on a sheet of paper or index card first. When you're ready to transfer it to drafting paper, I suggest putting the paper over your top to get an approximate size.
2) Pin the paper pattern on fabric folded in half (like the index card folded in the last step), trace it on, and then cut it out with fabric scissors.
3) The pattern should look like this once it's cut out. I placed my finished arm sleeves on the top (see the similarities?).
4) The top right of the X (circled) will reach the front of your costume. We will sew velcro onto the other side. (Alternatively, you can use sew-on snaps.)
5) We will sew the two blunt ends together. This is where your arm will slip through, so put the two ends around your arm, and see how far you will have to sew it in.
6) Add velcro to the other side on the top, and add corresponding velcro to the actual top itself. Sew the velcro in on the shorter sides.
7) For the armband itself, sew all the velcro pieces first. Then sew double-sided bias tape onto the endge of the armband. I used two packs. Sew skinny ribbon along the side. Draw the +C shape onto yellow fabric, and cut it out. I added no-fray glue onto the sides so it wouldn't fray.
Your armbands are done! If you have any questions, feel free to contact me at firstname.lastname@example.org or Facebook.