Ever tried sewing a jelly?

Its almost the same sewing someones mouth shut, especially if the tongue is trapped in between.

Especially if you do it properly; removing the lips first, to make the mouth flush with the persons face. The blood's easily dealt with, by spraying superglue on the surfaces, but it always looks better if you make the effort as the face ends up almost smooth from the nose down to the chin.

If you want to make sure your hard efforts don't get damaged, a couple of tiny screws in just the right place lock the jaws closed. 

Once everything is done and bandaged up, you can fit the helmet.

The helmet is special as well; stainless steel, a featureless face with nose holes only, and a spike on the collar with a tube in it that will give the wearer a tracheotomy when the hood is locked shut, so they can still have drink and liquid food. After all, they are meant to suffer, not starve to death.

Then, you give them a strong shot of painkillers and leave them to wake up on their own.

Its fun to watch the first moments, especially if you leave their hands free. They literally claw their fingers red raw trying to get the helmet off. They never will, you see...