It’s simple, talk to them!
The Proceedings of the National Academy of Sciences published an intriguing study of 120 babies from 4- to 12-months old, tracking their eyes while their parents spoke. At four months the babies gaze at their parents’ eyes. By six months, though, a dramatic change has happened: babies are intent on lip-reading. When someone speaks, they stare at the mouth -- puzzling out what this whole talking thing means and how it works. By 12 months old, babies developing normally have come to understand the mechanics and go back to watching the eyes during conversation -- unless unfamiliar sounds, such as a foreign language, come up. The eyes also dilate to unfamiliar words.
I think this is one of the reasons that speaking and reading to babies is so powerful. Use the opportunity when they watch your mouth during speech to read to your baby, so they can watch the same words in the same order again and again.
And keep in mind: Babies understand far more language than most people think. Using a tool like the KidGlyphs app can help you bridge the gap in communication and improve language development.