You add the sts, but your yarn is at the tip of the needle where you put the last CO stitch. So you have to knit them to in order to get to the original sts. A knit cast on is knitting into the first st on the needle, leave the st on and put the new st on the L needle too. Then repeat. For the cable cast on, insert the R needle between the first 2 sts on the L needle, wrap the yarn and put the new st on the L needle and repeat.