Though there will be a pocket bag inside, the finished double welt pocket looks very much like a large bound buttonhole.
1. Mark pocket position on right side of fabric with your choice of marking method - tailor's chalk, tracing carbon or machine basting would work.
2. Cut the welt strips 1" (2.5 cm) wide for a finished 1/4" (6 mm) wide welt and at least 2" (5.1 cm) longer than the desired pocket opening.
5. Cut the pocket opening. Cut down the middle stopping ½” – ¾” (1.3-1.9 cm) before end of pocket. Clip diagonally to backstitched points. Keep diagonal triangle longer, it’s easier to work with.
7. Inside pocket bag - cut the pocket bag the width of the welt plus double the depth you want the pocket to be.
8. On the inside, begin by stitching the (upper) edge of the pocket bag to the cut/raw edge of the top welt. With right sides together, begin on the welt extension, then as you stitch, you'll also sew through the fashion fabric on exactly the previous stitching. Be careful not to catch the triangle in this stitching.
11. Trim pocket bag width as desired. Then sew sides of pocket bag together including welt extensions.
If you've never sewn a double welt pocket, I highly recommend creating a sample from fabric that is lightweight, yet has a relatively crisp 'hand' so that when you press the fold in the welts and when you turn the welts to the inside they will press nice and flat.