I made a couple of pair of arm warmers. It was comically simple:

1) Go to cloth store and buy material. Knit, synthetic, warmish is what I got, though fuzzy would work, as well. I got (I think) 1 1/4 yards

2) Measure upper arm, wrist, and length of arm around a bent elbow. Add about an inch to the length measurement. You might consider writing these numbers down.

3) Draw a rectangle to those measurements directly on the fabric & cut out. Because the material is stretchy, you don't need to add ease.

4) Sew the long seam. (Serger, just shave the edges.)

5) Hem as pleases your fancy. You can slap some sport elastic on the top at this point, too. I did one pair with and one without, and they both work - but I ride a recumbent and my arms are up so gravity isn't pulling them down.

6) Pull 'em on and go for a ride. (Hey! They fit ME!)

Try it, you'll surprise yourself.