After the fiddly business of inserting the welts and opening up our pocket, we're now going to insert and attach the pocket flap and then the pocket bags.
First up, slip your pocket flap into place. It should fit pretty neatly, with the fabric side facing up and the lining side against the jacket. 
Now, turn your jacket over so you're looking at the wrong side. You should be able to see the raw upper edges of the pocket flap peeping through. We are going to secure the pocket flap to the upper welt. (It can be a bit confusing to tell up from down once you push the jacket body out of the way so make sure you double check which is your upper welt before you pin and stitch.) 
You want to arrange the pocket flap so that the line of basting holding those raw edges together is level with the line of stitching that holds the uppermost welt in place. To do this you'll need to grab the upper welt's seam allowance and make sure it's separate from the jacket, so when you pin the flap to it you're only pinning through the flap and the welt. 
Stitch the flap to the welt seam allowance, keeping your stitching as close as possible to the welt stitching line (but not crossing the stitching into the visible welt section or catching any of the jacket).
Now your flap is attached it's the pocket bag's turn. Start with the fabric pocket bag piece; keeping on the wrong side of the jacket, lay it over the upper welt and flap with its right side facing downwards. Match its raw upper edge to the raw edge of the flap. 
As you did with the flap, pin and stitch the pocket bag to the upper welt, once again keeping your stitching close to the existing lines of stitching. You will now have 3 separate lines of stitching on the upper welt: the line attaching it to the jacket front and then two further lines within the seam allowance, for the flap and the pocket bag.
You can see in this photo the 3 layers of the 3 pieces: bottom is the welt, then sandwiched in the middle is the flap, with uppermost the fabric pocket bag piece.Flip the jacket upside down (with the wrong side still facing up) so you are now looking at the lower welt. Attach the lining pocket bag piece to the lower welt in the same manner as you treated the fabric pocket, but matching the raw edge of the lining to the raw edge of the welt. 
You'll now have both pocket bag pieces attached and flapping free! 
Turn the jacket the right way up so the fabric pocket bag is now hanging down over the lining. The lining will hang down lower at the bottom. 
Next step is to stitch the two pocket bags together around the edges of the fabric piece (i.e. ignoring that excess lining at the bottom). You'll need to pull up the ends of the welts to ensure that you're not stitching through the main jacket body at any point. At each end of the pocket opening, behind the welts, you'll find little triangles of fabric pushed through from the right side of the jacket (these were created by your Y-shaped pocket opening cuts). When you stitch the pocket bags together, make sure you stitch through the base of these triangles. You may find it easiest to do the pocket bag stitching in sections. 
You may find it easier to stitch the two bags together in sections rather than one single bout.
You can now trim the pocket bags down to the same size. I used pinking shears to minimise fraying but you could also zigzag around them (the pockets seams might experience some abrasion depending on how much you use them – most of the seams don't need this treatment as they'll be safe within the lining). 
Well, that's that – your pockets are done! They probably look better than mine! 
Now that part of the marathon is over, we can look forward to the next section (published on Saturday), which is where we assemble the rest of the jacket body and attach the upper collar – and our work-in-progress starts to look like an actual blazer!