Hi, I know there are lots more on here with more professional experience, and hopefully they will be along soon... (my professional sign lang experience is with Deaf people, but my 'mummy' experience is with my son ASD and LD).
For us, as I am fluent anyway we started to sign whatever we were saying, (however we were speaking to him in 2 or 3 word sentences, so it was very simple) eg ds want milk? playtime finished. DS home now. etc
What was useful was;
We spoke at the same time as signing,
We recognised that reception comes before production. (So he could understand a sign before he used it back to us.) It was a few months of signing to him before he used a sign independantly without prompting, (we were in the supermarket and it was orange, i still remember
)
Signing about stuff he was interested in/liked/used, was important. milk, bed, chocolate, mummy, daddy, car, home, etc etc.
Making it fun, giving lots of praise/rewards for responding to sign and even more for using it himself.
Teaching the people around him to use it to...
there are lots of resources around, have a look on line, especially Forest Bookshop, (or pm me if that would be useful.)
Good luck and have fun! (using sign was a life saver for us, hence the name
)