My approach is to use an upright wooden post about a foot high attached to a wooden board. The post is padded with a few pieces of newspaper( not too thick because its got to be sufficiently thin to allow for the neck) and taped in place. Then the newspaper is covered with a plastic bag which is taped in place.
Then you start building the head.
The idea is to have the sitter infront of you perhaps in profile with their left ear showing and you build the equivalent profile on the stand as you start the sculpture. Then the sitter revolves to the right a little (they are seated on a chair on a revolving platform) and as they turn, you turn your sculpture and again reproduce the sitter from a different angle .
So you keep moving the sitter and the sculpture until you have the basics of the work in the round and then you just keep working on it joining up all the views.
It is a good idea to photograph both the sitter and the sculpture as it does help to see the sculpture through the lens and it makes it easier to see errors. It is also a huge help just to get other people to look and see what they think is odd or wrong and time is a help too. If you can leave work aside without having to work on it for a week or longer it is much easier to see what is wrong with it.
Once your work is done you let it dry enough to cut off the top of the head and scoop out the inside leaving a skin about an inch thick (which you will need to prick to stop air bubbles that can make it explode when it is heated in the kiln). Once you have scooped out as much as you can you pull the head gently off the stake and scoop out the neck and the shoulders.
Then you fire it.