Here is a YouTube video on making faces using a mold. The lady in the film is using a mold for a face about an inch and a half long, where we need something about a half inch in size.
There is no reason we can’t make a face of the appropriate size, build the mold, then fill the mold with clay and mash it into what we want for expression.
Or, we can build one of a kind faces using this video
As good as the tutorials are here and elsewhere, it helps to see the process on film.