I'm not sure how obvious this is, but when you want to add ribbing to a piece of work, no matter what it is, it's much simpler to chain the width of the ribbing you would like at the end of a row and turn your work sideways, crocheting on the side you would like your ribbing to be on. Crochet in between the rows that were worked for the main part of the fabric. Work the ribbing like a normal piece (ribbing works best in front loop only single crochet, in my opinion). This is a time-saver compared to sewing the ribbing on later.