We do it with our nine month old, but neither of the other 2 really caught on until around 10-12 months. I just picked a few signs to start with (mommy, daddy, more, milk, thank you, all done). Make the sign for baby, then lead her through making the sign - saying the word each time.
For example, when we are eating breakfast Abby finishes the Cheerios on her tray. I ask, "do you want more?" while making the "more" sign with my hands. I then hold her hands and help her sign "more" while saying the word again. Then I give her more Cheerios.
Good luck. We've loved the infant signing.
Tamara, blessed mama to:
Nathaniel (5), Susannah (2.5), Abigail (14 mo), Sprout (due 4/20)and 3 in heaven: Asher, Ari, and Issac.
http://www.picturetrail.com/pfamilygal