Sewing Rule: When sewing slash pockets always stitch the side triangle extensions to reinforce the side pocket opening
Once you've cut the slash pocket opening and flipped all the pocket layers to the garment's wrong side, you should have a triangle-shaped extension at each side of the pocket. These extensions are visible underneath the garment layer at each pocket side. The extension should sit on top of the pocket's welt edges or pocket bag edges (depending on the time of pocket application).
On order to secure the pocket opening sides, it is important to stitch at the base of each triangle extension catching the pocket layer underneath. Use a regular presser foot or zipper presser foot (if working with thick bulky layers) to straight stitch the triangle base as evenly as you can.
When the triangle extensions are reinforced as described above, the finished pocket sides should be secured flat featuring a clean, straight seam.
When sewing self-welt or bound pocket applications, the triangle extensions is stitched along the base in the process of completing the pocket bag.
Start by stitching at the base of the pocket's right triangle extension moving down toward the bottom of the pocket bag. Pivot the stitch and continue stitching along the pocket's bottom edge.
Pivot your stitch to the other pocket bag side edge and stitch at the base of the left triangle extension when arriving at the end of the pocket bag stitch.
Stitching the base of the two triangle extension when completing the pocket bag will ensure well-defined slash pocket edges that are durable and properly reinforced at each side.