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Basically, you’re going to pick up stitches on either side of the center seam. The pattern wants you to count out 26 rows, then begin picking up sts going towards the center seam. Pick up 20sts this way, then pick up and yarn over for 15 rows, crossing over the center seam and finally pick up 19sts.
It may help to mark the end of the 26rows and then fold along the center seam and mark the same point on the other side of the seam. That’ll ensure that you pick up sts evenly along the two sides of center.