Hi and welcome to KH.
Knit across 12 (of the 24) stitches. Then you turn your work and cast on 6 new stitches and then turn your work again (so now you would have 6 new stitches, as well as the 12 you knit before the cast on; you're only going to be working with 6 of these old stitches, so the other 6 old stitches are the "rem 6 sts on needle" that you leave alone). Next, you would purl 6 cast on stitches and 6 old stitches. Turn again and continue working on just these 12 stitches.
Hope that helps.