With A, cast on 56 stitches. Work in Stockinette stitch (knit on right side, purl on wrong side) until piece measures about 11 in. (28 cm) from beginning. 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.
Using the purl side as the right side, 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.