In our last post we stitched our facings and attached the Upper Collar. In this post we're going to deal with the Under Collar, then join the collars together and the facings to the jacket.
Firstly, attach your two Under Collar pieces along the centre-back and press that seam open. Now you're going to attach the Under Collar to the actual jacket in exactly the same manner as you attached the Upper Collar to the facings in the last post. Match the centre-back seams and your collar notches to the jacket shoulder seams, and match the dots. As you did previously, stitch along the lower edge of the collar as far as the dots, then clip into the jacket and pivot to stitch up the next side of the collar, stopping at 1.5cm (5/8 inch) from the edge. If you need reminding of any of the steps, refer back to the previous post for more detail. 
Now we're going to match the Upper Collar to the Under Collar, thereby bringing the facings to the jacket shell. Match the two collars along their raw edges, right sides together. Starting at the centre back, stitch the two collars together along the long curved edge. When you reach the end of the long edge, pivot and stitch down the next, short edge as far as but not into the seam allowance where the collars join the jacket/facing. If you haven't removed/dislodged your marked dots, you're stitching up to them again. Backstitch here to secure, and cut your threads.
Kindly just ignore the bit where I'd basically burnt off some interfacing with the iron... ;)Next you'll move the seam allowances out of the way and start stitching again on the other side, which is now the front facing and the jacket front. It'll be a short line of stitching. If I'm honest, at this stage the instruction diagrams display better than photos can how you should be stitching.
This shows where you should stitch first: along the curved edges of the two collars and then down the next short edge.
Then, as shown in this illustration, flip the seam allowances (the ones joining the collars to the front facings/jacket fronts) over so they're untouched, and restart your stitching in the front facings/jacket fronts.
Pivot at 1.5cm (5/8 inch) in from the corner rather than stitching off the edge and then stitch down the Front Facing/Jacket Front. 
When you've finished this stitching, go back to the top and clip into the curved edge of the collars and into the corners to reduce bulk. 
Understitch the collar curved outer edge seam allowance (pictured above, clipped) to the Under Collar.
Turn your collars and lapels (yep, that's what you've just created!) the right way out, using a blunt pointed object (I use knitting needles) to push out the corners. I'm keeping this post short and sweet because these steps are actually quite fiddly and you may need to spend some time getting them right.
Therefore it'll be in the next post that we'll understitch the Front Facing and secure our collar in place (because at this point it's free to flap!).