One word after the other. Often there is lots of backtracking and deleting. But essential, a book is written one word at a time.
So long as I write at least one word a day, I'll be happy. Clearly some days I'll get hundreds of words. On really good days, a few thousand of words. Other days I'll struggle to get one sentence out. But just one sentence is a few words in the right direction.
Message: even on a rough day, write. Just one word is that one step closer to publicatioon.