Fold the pocket in half bringing the short sides together with the right sides touching. Stitch round the edge leaving a gap along the bottom to turn the pocket through.
Turn the pocket through and press flat.
Place the pocket roughly in the middle of one side of the bag. Make sure that the folded edge of the pocket is at the top. Top stitch the pocket in place.
Stitch the side seams and the base of the lining together just like the bag.