You can see this stage, 'modelling', as a 'second pass' over the head.
Bosting sets the main masses and articulations: how the masses 'work' and hang together. Now we model what we've got: subdividing, consolidating, bringing out secondary forms. I'm starting at the nose end and heading towards the ears in a general sense but you can tackle the head moving from area to area all over.
It's important: