Usually that type of collar, the below-the-notch sections are contiguous with the front facing.
These pieces get sewn to one of your two collar pieces.
The two collar pieces are sewn to each other along the outer edge.
The front facing/ collar section gets sewn to the body of the jacket.
Then you press, press, press before hand sewing the second collar section to the now existing seams around the neck line of the jacket.
Yes, it is a complicated process. If what I posted still doesn't make sense maybe you can scan or photograph the actual instructions that are troublesome and I can try more direct translation for you.