Here's what you'll need:
A mouthpiece from an old snorkel or a replacement from a dive shop
A plastic valve that can open and close on a slide, such as a water flow regulator valve for sprinkler systems.
A zip tie or super glue.
Step 1: The mouthpiece
A diver should have a few left over snorkels hanging around the dive bag. Feel free to sacrifice one of these.
Or, it is prudent to visit you dive shop and pick up a replacement mouthpiece, making sure to thoroughly sample each one for that perfect fit.
I find that the LONG BITE works great for my horse face and adequately fits my bursting jowls. Be sure to choose a good mouthpiece because you will have this in your face for at least 30 minutes a day for the rest of your diving life.
I use Diver's Supply for all my online diving needs. I suggest you do the same.
Get ready to start doing impressive things like this.