This is just a small addendum.
Here is the finished nekkid head.
I pretty much insert the mouth like I’m putting in a mouthguard for a football player–open the foam mouth wide and shove in the mouthpiece(folded over with the cardboard facing out). Then it’s a matter of pressing the foam mouth over and onto the cardboard and lining it up straight.The contact cement at this stage is not willing to let go if you have something you need to re-align or shift, so you have to be fast.As you can see here, there’s a small gap between foam and cardboard on the lower jaw that I need to fix.
The addendum here, which I didn’t take photos of(wasn’t sure how it would look), is a re-adjustment of the nose. I thought it was pointing upward a bit much, so I cut a dart in the foam, right above the upper lip. Using contact cement to seal the dart together, I tipped the nose down a few degrees.