Fold the pocket back down and this will flip up the welt so that it sits flat on the right side of the garment.
Fold the garment over sideways to reveal the small triangles at the sides pf the pocket opening. Sew across the triangles between the ends of the placement lines, reverse stitching a couple of times to ensure the stitching is secure and then sew down the side of the pocket bag.
You can neaten the edges of the pocket if the pocket is going to be visible inside the garment.
Finally, catch the sides of the welt down, either by hand or by machine sewing across the welt.