Question: How do you plan your book chapter by chapter?
You will probably have some planning to do before you worry about chapters. You may start with characters, setting, or genre.
Once you get to plot, I think it's better to plan by events rather than chapters. It's easy to do, if you think of an event as one thing that happens.
A story, as Aristotle pointed out, is a series of events linked by cause and effect that show a change in the main character's fortune.
The difference between a story and a simple tale is that a story will have four main throughlines (overall, main character, impact character, and relationship), and possibly subplots, woven together like a braid.
Most writers find it burdensome to plot an entire book in advance, preferring to invent as they go alone. On the other hand, it helps to have some idea where you are going.
So, if you plot the 4 major signposts or events for each of the throughlines, that gives you roughly 16 major events - which is enough for you to always know what you are working towards. Of course, some or many of these signposts will actually be sequences of events, and you may need smaller transition events to take you from one signpost to the next
If you can describe briefly what each of your events will be, and write
them up in the form of a synopsis, so that they flow together well, you will have much of your plotting done. You can see easily if your story works.
You won't know how many words it takes to describe one event until you actually write it. That's when you can see where the chapter breaks should go. It's generally best to end a chapter at the end of an event, when you have the reader anticipating the next event.
You may have more than one event per chapter, if the events are short, or you may have one long event per chapter. The idea is to make each chapter a comfortable length for the reader, so that reading just one more chapter does not seem like a daunting task.
Something that helps a lot is to use index cards. Write a description of each separate event on a 3x5" card. Do one throughline or subplot at a time (perhaps use a different colour of card for each one). Make sure each of these plotlines makes sense on its own. Number the cards in each plotline so you know what order they go in. Then merge all these rows of cards into one big row or deck, in the order the events will appear in your book.
As you write, you simply work your way through each card in turn until you get to the end.