You cast on to the end of body width. You will make no increasing or decreasing, you will knit as long as you wish. You will knit 3X3 (3 plain and 3 purl) and make a rectangular piece and cast off/finish. For the arm part, beginning and finishing edges must have holes as big as you can get your arm in. Then you can stitch.
You can complete it with an appropriate button. If you like, you can use a stylish pin instead.