Here's a guide i found that uses a similar face setup to what I have for the models here, timestamped at the shader setup since everything prior has basically already been set up for you in the models themselves. This won't work as well on characters like berdly or susie's mouths, as they require individual UV editing, but for most of the facial expressions this should work fine.

https://youtu.be/E96m9Z4iTcc?t=349