I'd start by draping a mock-up on a wig head (preferably as close to your head size as possible), and making a couple test runs. I've made a few face masks out of spandex, and it's hard to get them to fit right even with the forgiveness of stretch, so you may want to think about making the back half of the head and the inside of the mouth in stretch fabric. That'll also give you the added benefit of being able to move your head and face.
Personally, I'd use seams down the centre of the face, across and through the mouth (like the picture), and right along the jaw line. I'd also make the neck separate to try and get as close of a fit under the chin as possible. Also, you're going to want to try it on a couple of times to make sure it fits well.
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