Leave the 19sts on the holder. If you read ahead, perhaps to the Finishing, you'll see directions for putting the sts back on a needle and adding an edging to the pocket top.
For now, take the directions for adding the lining step-by-step.
Knit 9, place the next 19sts on a holder, work across the 24 pocket sts (these must be on a holder or waste yarn or even another needle at this point) as follows: (p5, M1purl) 4 times, p4.
Then knit 44sts, and repeat the pocket lining 24sts with the second pocket as above; finally knit 9sts.
After this first row, the lining will only be attached by a single strand of yarn but as you continue the pattern, the lining will become more firmly incorporated into the knit fabric.