BO is bind off, same as cast off. The Drops patterns use cast off even when it's for one st, where we would use dec. They also say to cast off at both sides, which sounds like it would be on the same row, but you can't do that at the end of the row, you have to do one at the beg of the WS row.
Here's what it says for the neck:
"Dec. 1 st each side towards the neck on every other row 10-10-11-12-12 times and on every 4th row 5-6-6-6-6 times. At the same time when the piece measures 32-33-34-35-36 cm cast off 3 sts each side of the MT at the sides."
Work the neck decreases and then when the piec is the right length for your size, BO 3 sts on either side of the marker on each side. That would probably be 6 sts on each side. Then on the front only:
"Front piece: Continue to dec. for the neck shape. At the same time continue to cast off for the armholes each side on every other row: 3 sts 0-0-0-1-1 times, 2 sts 1-2-3-3-5 times and 1 st 1-2-2-3-3 times."
Do the armhole shaping at the same times as you continue the neck shaping. Cast off 3 sts xxxxx times, means to cast off at the beg of 2 rows. So first you bind off 3 sts on the RS, and 3 on the WS. Then 2 sts at the beg of the next 6 rows, then dec 1 st on both ends of the RS row 3 times.
And looking at the picture, it looks like the decreases for the neck start about the same time as the shaping for the armholes.