lazy-hazy:

Here’s a very sloppy like crash course about expressions that doesn’t even really cover it all. Practicing is the first big step, and research is a good definite baseline.

Look up expressions by real people, use a mirror and play around with narrowing your eye brows, making your mouth do funny things, look at how your nose crinkles and where else on your face wrinkles (like the forehead and the nose to mouth).

(source)

As well, look up cartoons and animation, see how they use lines to create expressions!