There is a lot of misinformation and inconsistent advice.
Will sign language slow down or stunt speech development?
There is no research to support that claim. In fact, research supports it benefits language development.
Recent studies have shown that learning basic functional signs may help with the prevention of behavior problems for children at risk. It may be easier for a child to learn sign language before they are able to learn motor planning for speech production. This ensures brain development continues responsible for language acquisition.
Using sign language will give your child the tools to help them functionally communicate. It may help ease frustrations for the parent and the child. It teaches children that their communication is very powerful. It allows the child to feel empowered, helping build their intrinsic motivation.
Pick a few easy functional signs to start with, such as more, all done and help.
Learning specific signs is easily obtained via the internet.