Place the welt between the bottom placement marks, so that the sewing line is between the two placement lines. Pin in place and sew between the placement marks, making sure that your sewing does not go over the welt onto the garment.
With both pieces right side up, lay the pocket facing over the pocket back, aligning the top edges. Zigzag stitch across the bottom of the pocket facing to attach it to the pocket back. Machine baste the top edges together, 6mm from the edge.
Place the pocket back right side down along the top pocket placement line, with the raw edges towards the welt and the sewing line in line with the placement marks. Pin and sew in place along the top pocket placement line, making sure the ends of your sewing sewing start and finish about 3mm inside the sewing line for the welt. This ensures that the welt sits over the pocket opening and there will be no gaps.