It's quite hard to explain. Quite often there are huge chunks of text which just literally need typing. That's more usual for non-fiction of all kinds, including this reply. More frequently for stories there is a kind of film - like an 'episode' - that I can play and replay in my head and then describe when I type. Plans tend to be silent movies and dialogue is added in the first (in-head)draft. Once I'm typing I might tweak vocabulary or sentence structure but the story flows. I can only say that the internal work is quite hard - it's just that it doesn't have to be done at a keyboard or with a notebook.