thread the tail of the yarn through the front of the fabric and secure on the WS. Insert the needle into the middle of the stitch to be picked up and under the top of that stitch.
wrap the yarn around the needle and pull through this part of the stitch. This is your first picked up stitch.
continue to pick up stitches in this way until you have the correct number. If you want your pocket to be stocking stitch, p the next row.