One element is to smile with your teeth. If a woman smiles with her mouth closed, it's just a friendly thing. Old gf told me that and a couple of decades of observation have born that truth out.
Know that face you make when you really want to smile, but are obviously holding it in? That can work.
Making strong eye contact, holding-it-in-smile, then blushing away, just to look again with a full-teeth smile sends a strong signal.
OTOH, if you have to consciously think about how to compose your face, you better be a trained actor. Humans have a solid sixth-sense for the disingenuous.
In any case, don't merely stare without expression. Yikes.