I signed for one of my charges. From about six months I signed a few basic words eg milk, dog, more, water, up, nappy, eat, please, thank you. She was able to sign back to me by 8 months and we added more signs a long the way. She spoke her first words at nine months "what's that". I left just after she starting talking but went and visited her recently, she is now three and speaks really well.
I ordered a BSL dictionary from amazon and picked the words to use.