Too many possible ways to go, need a better ride of what you want the head to look like, and what it has to be able to do.
What style - realistic, caricatured, cartoony, stylised, simplified?
What does it need to do - mouth open and close, or not? Eyes rotate? Eyebrows move to suggest a change of expression? Or have extreme changes of expression?
Direct methods: Rigid heads can be sculpted from an epoxy putty like Aves Apoxie, mix 2 parts and sculpt into shape. Or polymer clay like Super Sculpey or Fimo - model the head, bake it to harden, and you have a finished head. It can be very nicely modelled to look hyper-real, or any way you want it to look, people make amazing art does with this stuff. But will lack movement. You can make the jaw separate, attached with armature wire, if you don't mind there being an obvious gap. Or sometimes, if there are deep folds either side of the mouth, the gap can be concealed. Or, replacement mouths can also have the join hidden in the folds. If it is smooth face, you my need to smooth over the join with a matching coloured plasticine. Sometimes, people make a hard head with the mouth area smooth, and stick a plasticine mouth on, which can be animated. (Often looks like a doughnut.) Or therein be a hollow space where different mouths can be fitted. These days, different expressions can be designed on the computer and be 3d printed out, as they do at Laika. The older method was to hand sculpt several different mouths, or whole faces, that can be put on the head, photographed, then taken off and the next replaceable part put on.
You can also do a direct build-up with soft urethane foam, carved with scissors, glued over armature wire, and that is flexible so it can have movement. But it may look a bit rough, which may suit you if you want that look, or may not. The control over the shapes and surface is not as good as with a modelling clay or Sculpey.
If it needs to look very well finished, but also have some movement, then you may want to sculpt the head from plasticine, make a plaster mould, then cast the head in silicone or foam latex. But that involves more stages and more materials than a direct method where the stuff you sculpt is the actual puppet head you animate.
Very detailed and realistic heads in 1:6 scale are made for 12" tall action figures, and can be found on eBay. The are hard plastic, so there is no mouth movement. I made some bodies for film where the puppets were all a realistic 1:6 scale, and I found the heads very small, and difficult for me to sculpt in that size using the plasticine I have. A very hard plasticine would work better I think. My own puppets do sometimes use bought clothing in 1:6 but I make the heads bigger.
I have a couple of quick tutorials on Youtube, one showing a rough foam build-up head, the other a head cast in silicone with wire inside to move it. I wanted to at least be able to move the mouth when the character talks, for both of these. The Silicone head one shows a few different heads at the start, then goes through the process of sculpting, mould making, and casting. https://www.youtube.com/watch?v=5fWdZnQRzB0&t=1s