Here is an arm warmer pattern I made up using a diamond knitting pattern. This project is fairly easy as it is knitted on straight needles and all you really need to know are the basic knit and purl stitches.

I did not really test the gauge, but I know that I tend to be a loose knitter.

Materials needed:
You can play around with different kinds of yarn, I just used a normal Red Heart medium worsted yarn in lime green.

I also used US 8 needles (Takumi US 8 Clover needes, made with bamboo, they are my absolute favorite needles to use).