Since you won't be knitting the first or last 18 sts, you start the center sts with what would be the 19th st of the pattern. Rows 1 and 3 are just knit, and since they want you to k18, then you'd probably be starting this section with one of those rows. So you just knit the 30 sts, then turn and begin row 2 or 4 with either k1 if you're on row 2, or sl1 if it's row 4. You need to keep the sts the same as if you'd done the other 18 on both sides. While you knit the 1 inch of the stitch, pay attention to how they look so you can keep them going in the pattern.