Face your baby closely while they are alert. Make one slow, exaggerated movement — mouth wide open, or tongue out — and hold it.
Wait ten seconds without changing. Imitation at this age is slow, and most parents give up before it arrives.
When they attempt anything similar, react warmly and do it again. Then copy something they do, so the imitation runs both directions.
Imitating a face requires matching what is seen on someone else to a movement felt in one's own body, without ever having seen one's own face. It is a foundational social skill and an early sign that other people are being processed as like me.