Place pocket bag on bottom, right side up, next place garment (with fused interfacing applied to wrong side under welt), right side up, then prepared welt (with fused interfacing and box marked with stitching lines)
Stitch both long ends of drawn box and backstitch at the vertical line. Be sure to stop EXACTLY at the corner of this box.
On right side, press both sides of the welt toward center.
On wrong side slash welt open and, using very sharp scissors that cuts to the point (like knife edge) make diagonal clip to each corner.
Pull welt to inside.
Press to form welt. If unsure about this step refer to Fashion Incubator's blog entitled How to Sew a Single Welt pocket.
Right side will look like this after pressing welt.
Add facing to pocket bag. You will need to determine measurements based on the size and location of your pocket placement. I like to use an opening of about 4.5 inches for women's pants.
Stitch, vertically, the ends of the box from this angle.
Either ditch stitch here or...
Edge stitch and add buttonhole at this stage before finishing edges of pocket bag.
Pull pocket bag through to right side of garment.
Pulling to right side.
Stitch sides of pocket bag, right sides together.
Pull back through to wrong side, press seams, stitch again about 1/4 inch from edge enclosing previous seams.
The welt and all of its unfinished seams should be enclosed in the pocket bag.