I teach my baby myself. I searched the signs on internet. I believe that babys parents are their best teacher. Just do the signs repeatedly. But it takes time for them to understand and sign back. I started signing to my bb since 8months and she csn sign please, thank you, milk, finish, some more, diapers.
I believe that bb wants to express themselves before they can do so verbally. Teaching her sign makes her feel very happy to b able to express her idea and also willing to learn communicating with you both verbally and non verbally.