Get face to face with your baby when they are alert and content — not hungry, not tired.
When they make a sound, wait until they finish, then copy it back as closely as you can. Then stop and wait.
Most babies will go again. Keep the exchange going as long as they lead it. If they look away, the conversation is over and that is a complete result.
Copying a baby's own sound teaches that vocalising produces a response from another person — the single most important discovery in early communication. The structure of alternating turns is being learned here long before any word arrives.