Adding on stitches

I have an afghan pattern that is worked diagonally. You knit 12 sts for 24 rows. Then you cast on 12 sts, k to end. Next row: Cast on 12 sts, k12,p12. k12. It's worked in blocks of 12 stitches. I need help casting on the stitches on work that's already been started without getting extra yarn where the two meet.