I learned that age 6-9 months is when babies can start paying attention to what you sign. You should say the word while doing the sign to help them learn. Although babies may not be able to sign right away, they can at least recognize your signs and meanings. At some point, they'll be able to sign back at you, but can only learn 1 or 2 signs at a time. So, if learning is slow...don't give up...it's normal. Babies have ideas to communicate before they're able to talk, so sign language allows them to express those thoughts.
I love the flash cards. I've started a couple of signs with my 7-month son. I leave the flash cards on the coffee table so that our nanny and other family members can pick them up and use the same signs that I'm using with Michael.