Ok lets think about what you are doing and what the patterns says.
first, does the pattern have a schematic, (a simple drawing of what the finished peices is supposed to look like?)
if it does, look at it.
2 --the directions say "to dec at end of next and alt rows "
well to me, NEXT AND ALT(ernate) ROWS means rows 1 AND row 2 (and rows 3 AND row 4, and so on)
which makes sense, since the decreases, which follow "casting off 10 stitches on each edge" are just a continuation of the shaping for the armhole.
and since i presume you have 2 arms, and you are shaped symetrically, you want your sweater to have armholes that are shaped symetrically!
the bottom of the arm hole is formed by the 10 stitches cast off, and then there is a small curved, and then the armhole continues straight(usually right up to shoulder cast off.--in this case-10 more stitches, on each side side)
as you work up to the shoulder, you might have a center neckline shaping to do (and again, do it symetrically) and shoulder shaping to do, (again, symetically!)
you might even have the dreaded "AT THE SAME TIME" (make slow flat shaping for the shoulder by binding off 10 to 12 stitches at a time) combinded with steep curves (bind off 1 stitch ever (or ever Other) row.
(and the other side will be shaped by first making the steep curved neck shapping and at the same time, the slow flat shaping of the shoulder)
But don't worry. think about what you are doing. Look at schematic if available (and if not, smooth out a T-shirt on a your bed, and look how its is shaped--at the armhole, at the shoulder, at the neck--(look at the back of course) and then look at the front.. (almost the same.. except for the neck.. and you swearter might very well be the same! )
UNLESS YOUR SWEATER IS DRAMITICALLY ASYMETRICAL--your shaping will be 'balanced' (what you do on one side/edge, you do on the other side/edge.)