When you walk in the hall & are about to pass someone, look at them lightly and smile. When you're speaking with someone, instead of just listening and nodding, smile slightly sometimes. If you're in a lecture, smile when you notice the lecturer's eyes scanning towards your portion of the room. After saying something non-negative, smile as the other person starts talking. I'd say it's about smiling slightly when one catches another's glance.
This isn't a big, cheesy, ear-to-ear grin, but a genuine, small, slight smile. The ear-to-ear ones tend to come across as creepy or mocking or such (assuming you're not doing it to someone in a humorous moment). Smiling doesn't have to be a huge, but only enough to keep your mouth from looking frowny. For me, that means my mouth looks basically flat/straight across and my cheeks pull up slightly.
I got called out for this once at a retail job where a fellow worker came up to me and said something like, "I never see you smiling! Do you smile? Smile more! Life isn't so bad." I stopped smiling, looked right at him, and told him, "This is me not smiling. I have a naturally frowny face. The only other smile I have is this." Then I did the creepy big pulled-back smile. He was a bit taken aback but shut up (and sorta apologized).
PS I'm assuming you mean "When People Say Smile More What Do They Mean?"