The 18 stitches are your "knit 17, k2tog"
You then pretend your at the end of the row,
turn your work and knit back again.
But before you start knitting back, put all the stitches you didn't get to (after your 18) on a stitch holder or spare needle.
So now your knitting back to the side edge.
Then you knit back to the neck edge and decrease again.
Turn it around, decrease again, then knit back to the side edge. (So that's your decrease for next 2 rows)
Then you just decrease on every second row.
For now, you're just ignoring all those stitches which came after the 18.
Hope I didn't make that too complicated.
Best of luck!