Row 20: Knit 9, beginning with next 2 stitches bind off 21 stitches, knit across remaining 9 stitches. (This includes 1 stitch already on right needle after the bind-off.)
Put first set of 9 stitches on holder.
Rows 21-23: Knit 9 stitches on needle.
Bind off all 9 stitches; cut yarn.
Rejoin yarn where you started binding off 21 stitches. Cast on 12 stitches using cable cast-on method.
On row 20 you want to end up with 9 stitches at each end and 21 bound off in the middle. Then they want you to put the first 9 (before the bind off part) on a holder to work later, and to work rows 21-23 over the other 9 stitches. After you work those 3 rows you bind off those 9 stitches and cut the yarn.
That next part about "Rejoin yarn where you started binding off 21 stitches, " is a little unclear to me. You usually start knitting to "join" and they just say "join" and cast on 12 stitches. I wonder if it wouldn't work to put the other 9 stitches back on a needle with the spot next to where you bound off the 21 right next to the working end of the needle, then put the needle in your left hand and do a cable cast on of 12 and go from there.