"Working sloped bind off,cont as foll: (1 st. at the beg of foll 4th row once (3times), 1 st. st beg of foll 6th row) 9 (5) times. 1 st at beg of foll 4th rowtwice--21 (23) sts dec'd for nect."
Instead of BOs, use decs, they're smoother. Dec 1 st at the beg of the 4th row 3 times, then at the beg of the 6th row 9 or 5 times, depending on your size. Then 1 st at the beg of the 4th row twice.
That's all at the beginning of the rows, at the end of the rows you'll be doing armhole shaping starting when you're 8½" from the beg or cast on. Switch to the other needles and do the armhole shaping on the WS rows, however you did it for the back.