Working both sides at the same time with separate skeins of yarn, at each neck edge bind off 3 sts 2 times, 2 sts 4 times...
You use separate skeins of yarn because the shoulders are separate. I assume that you've done some kind of neck bind off or put them on holders, whatever the pattern said. Your original skein should be already attached. You'll need to attach a ball for the other shoulder, but only once.
They want you to bind off three stitches at the neck edge on two rows, then two stitches on each of 4 rows.
Since you can only bind off the neck stitches at the beginning of the row, just bind of when you're starting at the neck edge.
So on the right shoulder of the sweater--the right side as you would wear it--you would bind off the three stitches, finish the row. Work back. bind off the next three, finish the row, work back. Bind of two, finish the row, etc.
On the other shoulder, you still bind off only at the neck edge, but you will be doing it on the purl side if you did the first shoulder on the knit side.