So the pattern says
Row 1 [WS]: CO 8 sts, p to end. 18 sts.
Row 2 [RS]: CO 8 sts, k to end. 26 sts.
Your've just finished a RS row and placed some sts on hold while you start to knit back and forth. Take the needle with the sts in your left hand and cable or knit cast on 8 sts. The working yarn will be at the tip of the needle when you're through casting on. Purl the cast on sts and the sts that were on the needle originally till you get to the end of the row. Now switch the needle back to the left hand again and cast on 8sts. Then knit across the cast on sts, knit the original sts and the cast on from the previous row. If you need to see cable cast on or knit cast on go to the Free Videos tab at the top of the page, Cast On (although it sounds like you've already learned multiple cast ons).