Use 8 ply, and 4mm needles.
Cast on 80 st, and work rib: (k2, p2) for 10 cm.
Next row, (k2tog, p2tog), rep to end. (40sts)
Next row p.
Next row k2tog to end. (20sts)
Next row p.
Next row k2tog to end. (10sts)
Slip rem. sts off needle and run a thread throughand secure.
Sew up.