To get started, get yourself a pair of size 3, 5" circular needles or you could use size 3 double pointed needles.
Cast on 30 stitches* of worsted weight yarn and knit on the round as many rows as you would like the cuff to be. I personally checked it to my wrist as I knit and ended with 20 rows, but you could go shorter or longer.
Next, you can either change yarn and color or continue with the same yarn. It's all according to your personal preference. I changed yarn to a green worsted weight. Now you're knitting the space between the cuff and the thumb hole. I knit another 17 rows here before starting the thumb hole, but measure it to your own hand and decide the amount rows accordingly.
When it's time to start the thumb hole, all you have to do is turn your knitting around and purl the next row all the way around to the start of the thumb hole. Then turn your knitting around again and knit the next row all the way around to the start of the thumb hole. Continue this turning/purling/turning/knitting until you have the complete thumb hole. It took me about 11 rows.
When the thumb hole is complete, continue knitting on the round until the arm warmer is finished. It took me about 9 more rows.
Cast off, sew in the end strings and knit another! It's that simple!
*If 30 stitches feels too small for your wrist, or you are using a different weighted yarn, just cast on the 30 stitches and knit on the round for a few rows and measure it to your wrist. You may need to start over with more/less stitches.
