This is good advice but with a caveat: some of the muscle control required for particular signs is not able to be learned by children until they're a bit older.

For example, voluntary supination/pronation of the forearm is generally not something a 9month old can do. If you try and teach them a common sign for "enough/finished" (fist closed, thumb pointed out, then rotation of the forearm back and forth), or "done" and "more" in the parent link, they probably won't be able to do it properly. They can copy something close to that (thumb out and wobbling their hand around? good enough!) so you have to go with the flow.

There are quite a few signs like that actually, so try and think about how many muscles move together, and how controlled or complex that is. Simple stuff is good -- and doable.