With A, cast on 56 stitches. Beginning with a knit row, work in Stockinette stitch (knit on right side, purl on wrong side) for 16 rows.
Change to B and work in Stockinette stitch for 6 rows. Change to C and work in in Stockinette stitch for 20 rows.
Change to B and work in Stockinette stitch for 6 rows. Change to A and work in Stockinette st for 16 rows. Bind off.
Weave in ends.
Wash by machine on a hot wash/cold rinse cycle with detergent and several pieces of clothing to agitate. To felt additionally, dry by machine on a regular setting until almost dry. Remove from dryer and pull into shape.
Fold piece to about 6 x 8 in. (15 x 20.5 cm), with a 3 in. (7.5 cm) flap. At center of flap, cut a slit for the buttonhole.
With B, join side seams using Blanket stitch. With B, work Blanket stitch around flap. With sewing needle and thread, sew button opposite buttonhole.