EZ Baby Suprise Jacket confusion

I need some help! I’ve gotton to 158 sts and am instructed to work on the center 90 sts for 10 ridges ( or something like that). I need some hand holding here-how do I work on the 90 sts?

You know where you marked the 36th and 125th stitches? Work across to the second marker, then turn and work back to the first marker. Work back and forth on just these 90 stitches for 20 rows–that will be 10 ridges.

Well, that is exactly what EZ says but you saying it, now it suddenly makes sense. I am not totally comfortable with EZ’s sparse instruction but they are good for me in the long run. Thanks!