ahhhh— this one’s a bit tricky to explain since the way each artist draws facial features change between person to person, and regarding the kind of character they’re drawing. everyone’s going to go about things differently to everyone else— but that’s what makes art so great and fluid between each artist.
but i can show you a tiny thing of what mine look like, at least?
starting off with the generic nose:
this of course changes depending on the character, their age, facial structure, gender, race— and so forth.
for mouths, it obviously depends on what expression the character is wearing. however, you could really simplify it into three dots that you can draw the mouth across. one for the center, and one for each side.
with an open mouth, i always start by drawing the upper lip before working my way to the rest of the mouth shape, and then the tongue and teeth. depending on the perspective and expression, the lower teeth may be visible too. play around and see what looks best!
i hope this helps a little!