Which ones?
For smiling/happiness, it's a little like squinting in the sun. For a more confident smile, keep the middle of your eyes open and focus the squinting on the outer edge of the eyes.
For frowning/anger, it's bringing the insides of your eyebrows down and towards the nose. Then raise the middle of your eyebrows a bit for worry/sadness.
For boredom, de-focus your eyes and perhaps let the eyelid drop a tiiiny bit, but just in a relaxed way, don't force it. Exaggerate your boredom by then raising your eyebrows a bit.
The above does not come from any research. I just played with my face then typed up what I'd done.