Well there’s really not much to it really. Spoken language is just a natural thing that people do. Even if you tried to keep signing going, your baby is still going to transition passively into speaking. If you’ve been saying the words as you’ve been signing them, they’re already well on their way to swapping one for the other.
By 18 months your baby is probably saying a few words already and this will increase to about 50 words by their 2nd year. Once they reach this milestone words will be added exponentially. Their mechanical development in their vocal cords coupled with their motor coordination will permit them to speak words that were too complex previously. Until a baby can say the words, they’ll just sign them. That’s the beauty of baby sign language – it’s used just as long as it’s useful.
Baby sign language is a stepping stone to speaking.