Quoting D♥'sK:" (For your child) How do you start it? What age do you start it?"
start with simple commands like please, more, eat, etc to express common needs i started with my kids at 5 months and they caught on quick a childs brain absorbs info quicker than an adults does (i took child development for 2-3 yrs) :)
DS started at 15 months and now knows 15-20 signs 5 months later. He's in speech therapy because he doesn't want to talk. We started out with one sign: more. We would give him one cracker or snack and ask him if he wanted more, and did the sign ourselves, then we did his hands into the sign for him and would immediately give him more. After a couple days he figured out if he did the hand gesture he got something he wanted. Once he was using "more" frequently we introduced milk and juice and he caught on to those very quickly. Now we only have to show him a new sign once or twice and he knows it :) I'm going to start sign language very early with DS2. 3 months or so, just showing him the signs when I talk.
My parents taught me sign when I was a kid. My mom said they started it with speech. So as I was starting to talk they started to integrate signs. Basically, just talk and sign at the same time. That's what my SO and I are going to do with our son.