mark top two corners for welt position 
iron interfacing onto inside of garment central to the marked welt position
Press under 1cm of the long edge of pocket bag facing & topstitch onto back pocket bag, make sure top edges are lined up together.
Iron interfacing onto front pocket bag , this folds up to create the welt.
Sew the bags onto the right side of garment as below, the back pocket bag is the upper one & the front pocket bag & welt the lower one. Make sure right sides are together. Precision is crucial so mark the 4 corners of the welt onto the bags & sew between the two upper dots & then the two lower dots remembering to reverse stitch at the start & end of each line of stitching to prevent unravelling.
A close up of how it should look, 2 parallel lines of sewing starting & ending precisely at each corner.
Only cutting the garment (not pocket bags) cut centrally between the 2 rows of stitching, stop 1.5cm short of each end & slash into each corner right up to the last stitch.
how it looks inside of garment 
right side of garment
Pull both pocket bags through to the inside of the garment, fold back the triangles to form the welt ends.
Flip upper bag down, the lower bag is longer because this forms the welt in the next step.
Manipulate the front pocket bag upwards to form the welt.
To secure the welt ends in place, stitch down the triangle through welt & back pocket bag. Continue right round sewing both bags together to the end of the welt on the other side.
Both pocket bags sewn together.
Overlock if required.
The welt pressed & finished.
Garment Construction 