Help! Casting on additional stitches?

I’ve just started making the Little Girl’s Sundress from Knitting Pure & Simple and I’m not sure about a direction. The dress is knitted from the top down, beginning with the straps. You are to make a 7 stitch strap and break the yarn and place on a holder. You then make the other strap the same but you don’t break the yarn or place it on a stitch holder. Here comes my question. The next line reads - Cast on 9 sts, then work in rib across the other strap. You will now have 23 sts. How do you do this? I’m guessing that after you have correct length of the strap you just start casting on those additional stitches but what is the best way to do this?

Kell Bell
Coldspring, TX

You could do a cable cast on. A video for it is on this page: