Every time I create a new character I'm faced with the potentially daunting task of generating dozens of mouth shapes. Sometimes I carefully draw out each shape, but I always prefer to use the technique below to get good, fast results.
When your making mouth shapes you want to be careful to keep the shapes on model. That is to say that you don't want an "M" that is more narrow than your "A". You also don't want the upper teeth moving up and down as you cycle through the various shapes. To that end I create all of my shapes using vectors and masks and I draw them all in place. Check out this video for more details.
Get the puppet and source files here.
https://www.mediaresonate.com/ch-mouths