THIS is something very hard to explain to be honest. This is like another level of character drawing even I’m still trying to get it right ;v ; (tho some people get it right at first then learns correct proportions after while others go the other way around, it’s a cartoon thing) But a tip that helps me a lot with this is to “act it yourself”—think about the expressions well and feel them, get into the story/scene, etc. For some reason, it will come out when you draw, like don’t be afraid to exaggerate your expressions. Trust me this took me a while to learn and even now, I’m still unsure if my expressions are right, if they match the proportions or if they’re too much over acting.