I'd like to mention again that besides first hand experience from signing with two babies, most of my information comes from this book.
Whether or not babies have the memory required to learn signs very early, I don't really know. But it doesn't matter because before around 6 months, they just don't have the strength and control required to make signs with their arms and hands.
So you can start signing earlier (like I did with Sophia) but don't expect them to start signing back until at least close to 6 months. (Super Sophie started signing "milk" and "potty" just shy of her half birthday)
If you do wait til 6 months like I did with Isabella, you still can't expect baby to sign back immediately. It takes baby a while to figure out what the heck mommy is doing with her hands. I think Bella was signing back around 7 1/2 months, but I don't really remember. Bell got really good at it really fast once she figured it out, whereas Sophie is really just now getting into it (she just turned 8 months) which is probably because now that I have two kids and a puppy, I'm just not as good about signing to her as I was when I only had one baby.
Once baby starts realizing that a certain action follows a certain sign, and that you respond appropriately when they sign, they will be eager to learn more.
So here's how to do it: You just...do it.
Using an American Sign Language (ASL) dictionary, (or just find out signs onlline) Pick 2 or 3 signs at a time, and do them every time you see that thing/action, and also SAY the word. For example, when you're feeding the baby, you might say "Milk! Baby is drinking Milk, do you like Milk, baby?" and be sure to do the sign every time you say "milk."
If you see baby noticing a picture of daddy, sign daddy, say "daddy" etc.
You can even do the sign "on" baby. Meaning that for signs like mommy and daddy that involve touching your face, occasionally sign it on baby's face.
I don't add more signs until baby initially starts signing back, (which, as I said, may take a month or so in the very beginning) but once Baby has the general idea, you don't have to wait until they're great at every sign to add another sign or two.
You should, however, only add a couple of signs a week, for your own sake. You need to be consistent, and it's impossible to remember to sign everything if you add too many at a time as you learn them yourself.
It is also really important, especially in the beginning, to make an effort to do what baby asks so that they will know that signing works, and that you understand. So let's say baby signs "milk," be quick about letting her know that you understand what she wants. If you don't respond to her signs, she won't bother to do them. Of course, you can't always "obey" when a child demands something, but just let them know you understand. Maybe your 10 month old keeps signing "outside" and it's way past bedtime..You can say/sign "outside" when you say "mommy sees that you want to play outside. See the window? It's dark now. We will go outside tomorrow." Kids are smart. They understand these things.
So really, that's it. That's how you do it, but here's a few more tips
- You should stick with official signs using whichever Sign Language is used in your country. Sure, you can make up your own signs, but what if you forget what sign you used for "ball" and you can't just grab your sign dictionary to remember what it was? So sometimes it's ok to make up your own, but I wouldn't recommend doing it for everything
- To keep baby interested, when you add a sign that YOU want baby to learn, be sure to add a sign that HE wants to learn. So some practical/convenient signs for you might be hurt, milk/nurse, potty/diaper, more, eat, mommy/daddy. Babies seem to love everything about outside, and some of Bell's favorite signs were: Book, ball, dog, cat, tree, outside, moon, car, hear, see, airplane, hat, glasses.
- Be sure to make sentences once baby starts learning. "more cheerios" or "see cat outside?" are some possibilities.