Have you ever encountered an on-seam pocket with a pocket bag layer that keeps rolling to the outside of the pocket opening edges? This occurrence is most likely due to the fact taht the front pocket layer (comprising the pocket bag) was not understitched prior to completing the on-seam pocket application.
Sewing Rule: When sewing a conventional on -seam pocket (using separate pocket bag pieces), you should always understitch the front pocket layer prior to aligning and stitching the pocket bag. An understitch is a straight stitch that is applied through the pocket piece and all seam allowance layers underneath, right next to the seamline. When the front pocket edge is folded, the understitched pocket piece remains flat and stably enclosed on the inside of the pocket bag.
Applying understitching is fairly easy: Once the front pocket piece is stitched to its corresponding front seam edge, iron the pocket layer and all seam allowance layers underneath toward away from the garment (toward the pocket piece) as shown above. It is easiest to iron from the right side of the seam.
Next, stitch next to the seamline through the pocket piece and seam allowance layers underneath. Apply this stitch as evenly as you can keeping all layers smooth and flat in the process.
In the finished on-seam pocket application, the seam allowance layers and the pocket bag are ironed toward the garment's front. This means that the pocket bag layer corresponding to the back seam edge remains flat while the front pocket bag is folded under creating a folded edge. If the pocket layer underneath this folded edge is not understitched, the pocket layer tends to roll toward the outside of the pocket opening and not maintain its structure properly.