I would first create the two extremes – mouths fully closed and fully open, then maybe 4 or 5 in between to give smooth movement,
Organise these stills on timeline using trial and error to get roughly the right speed from closed to open, then back to closed. Make a movie of this “cycle”, then this can be used repeatedly to synch to the lyrics – if a faster or slower syllable occurs, Ctrl drag the edge to speed up or slow down.
This is just a rudimentary method – you can get more sophisticated by creating different shaped mouths for “eeeee” or “ooooo” sounds, but that’s up to your artistic inclination!