Babies are absolutely fascinated by the world around them, and nothing is more interesting to them than your face, and what it can do. Not only that, but believe it or not, your baby really likes to copy you and they like it even more when you copy them!
Hold your baby close while sitting in a comfortable and quiet (distraction-free) place and while watching her mouth closely, imitate her sounds. Switch roles after a while and make obvious gestures such as opening your mouth wide or turning your head. Give baby time to respond, and she will!
Another variation on the copycat theme is to bring in mirrors to enhance their learning. Stand as close as possible to a mirror holding baby in your arms. Smile and point out what baby is doing with their face. Are they smiling? Point out the smile and say “look, your mouth is smiling” then copy them and say “look, now mummy’s mouth is smiling” pointing to your own mouth.
You can vary this by using a hand-held mirror, or by placing a large (no sharp edges) mirror on the ground for baby and you to lie on your tummies and look into.