As it is one of the trickier aspects of putting together one of my male turian mask kits, I took a series of close-up photos to show how that part looks on my sample turian (Victus). Hopefully this will be a helpful resource for anyone putting their own kit together!
The mandible and upper jaw both curve at the top and fit together at that point.
The mandibles form a sort of oval shape around the jaw and have a small gap at the front between the ends of the mandibles and the lower jaw. You will probably need to build up the area underneath with some epoxy putty to get the correct placement. This also helps secure the mandibles in the correct place.
Here are a couple of shots showing how the mandible placement looks from underneath.
When everything is placed correctly, you can see how the faceplate fits neatly into the gap left by the mandibles.