The instructions want you to work 3sts in the rib pattern (either knits or purls) cast off 1 stitch, work 2sts, cast off 1 stitch, work 3sts.
Cast off in rib means to work the 2sts that you need for the bind off according to the pattern columns of sts that you have been doing.
Same goes for the 4th row except you're casting on:
work 4 sts, cast on one, work 3sts, cast on one.
Just be sure to maintain the overall rib pattern. You may have decreased sts but follow the columns of sts (instead of 6x2 you may be onto 5x2 for a small area for example).