From the wrong side, carefully cut along the centre line, making sure you cut through the garment layer only. Stop 1cm from the end and snip into the corners to create a Y-shape. Make sure not to snip through your sewing., but get as close as you can.
Pull the pocket back through the opening to the wrong side of the garment and adjust it so that it sits flat.
Fold the pocket back down so the bottom edge lines up with the welt seam allowance.
And sew along the seam line directly on top of the first row of sewing.