I've been looking at all sorts of arm warmer patterns of various complexity. I ended up going with simple, without using a pattern at all.
I used a 4.00mm hook again - this size is becoming my favourite, it seems. I have no idea what the yarn is. I got it from a fabric and yarn exchange a couple of years back. With only a partial ball, I had no idea what to use it for until now. Whatever it is, it was pure luxury to work with! I think it might actually be a silk blend of some kind. Whatever it is, it's smooth, soft, and has a wonderful sheen.
The arm warmers are 40 stitches long, worked side to side in hcd in the back loops only. I ended up doing 24 rows before closing it up. It took several tries before settling on where to put the thumb opening. The opening is wider than usual, but it felt more comfortable on the hand that way. I ended up closing it up for 8 stitches along the palm, then leaving a 12 stitch opening for the thumb. I worked the seam by ss the sides together, so when I got to the opening, I continued to ss into the foundation chain only for the 12 stitches, before joining the two sides up and continuing to the end.
One of the beauties of working side to side in the back loops is that the resulting fabric is quite flexible. In this photo, my 13 yr old is wearing them, but the fit me, too.