Bind off 6 sts at beg of next row, work 12 sts, join 2nd ball of yarn and bind off center 28 sts, work to end. Working both sides at the same time, bind off 6 sts at beg of row, work to end; bind off 6 sts at beg of next 4 (4) rows, then bind off 0 (0) sts at beg of next 0 (0) rows.
BO 6 sts (RS). Work 12 sts in pattern. Join the new ball of yarn (just pick up and start knitting or tie loosely to the first st). BO center 28 with new ball and work to end of row. Turn, BO 6 sts (WS), work to center in pattern. Drop this ball, pick up ball for other side and work across that shoulder. Repeat this process 4 more times (2x on RS, 2x on WS). You'll have BO 18 st on each side (3x6 on RS, 3x6 on WS).
ETA: your BO edge should look kind of like this (X=BO):
If you want to avoid the 'stepping' don't work the last st on the row before the BO (just slip it to right ndl purlwise). Turn ndl to the BO row, slip the unworked st to your right ndl along with the next st, work the BO (1st of 6 completed) and then work 5 more. This will give a gradual slope.
Example of stepped shoulders here
and how to seam if you opt to do it that way.