Basically, you can just work it the same - 56 sts works for k2 p2 ribbing in the round or flat. Follow the decrease rounds exactly the same, first decreasing in every other purl rib, then a round with no decs, then dec in every other knit rib. You might want to do another dec round however, 28 sts on size 13s are quite a few sts. At that point you have k1p1 ribbing and can just do another plain round after row 31, then a round of k2tog leaving you with 14 sts which you can fasten off.
sue- knitting heretic