I know that when you get a book idea, you want to start writing immediately. However, there are some steps to do before you actually start writing. For some of these steps listed, there is another page to read more into detail if you're interested.
Routine
If your dead serious about writing, you need to have space set aside for it. I mean, sure, I guess you could write whenever you have the time, but sometimes you wont have spare time for your writing and you won't get anything done.
So, have designated times for your writing and have a goal for each session so you can get things done.
Find your audience
Before you write your book, you need to know who your writing for. Like, its not like your writing a romance novel for preschoolers. But know the age limit because you want your writing to draw in a certain category of people. It could be for age categories, but also the theme. Like, are you writing for middle school fantasy lovers?
Because if you write a book for the wrong category of people, they won’t be interested and your book will end up as a flop.
Plot, structure, and POV’s
These are the main parts of making the book. You need to know the plot, what happens, but you could still think of the plot while writing. But, you should have some key parts or scenes down so you know which direction your going in. Again, writing is fluid, which means that this is your book and there is no right or wrong way to write it.
For your structure, you should know the timeline of your story. How long is this book going to span out? Hours? Days? Months? Years? For this problem, I usually make a physical timeline with specific scenes planned out so I know when things happen.
And finally, know your POV, or Point of View. This is where your story is narrated from. First-person POV means the reader is the main character, and most authors use this POV because, in my opinion, its easier to do. And, it helps the reader get to know the main character better because they can see their thoughts and feelings. Second-person POV is when the reader is in the story themselves. It uses the word “you” a lot because it is the readers decision. This POV is used a lot in the “choose your own adventure” books. And then there is the third-person POV. This POV can be divided into two categories, third-person limited and third-person omniscient. Third-person limited is when there is a narrator narrating the story and the narrator only focuses on the thoughts and feelings of one specific character, or the main character. Third-person omniscient, however, also has a narrator telling the story, but the narrator knows everyone's thoughts and feelings.
So, once you get those things down, you can start thinking about the characters.
Get to know your characters
I know you guys might think this is stupid, but I’m dead serious. You think you know your characters by making their character sheet, knowing what they hate or love, or even knowing what they sound like, but you really don't unless you know how they would react in certain situations and how they talk.
What I mean by how they talk is like, what words do they use a lot? Do they talk sarcastically? And if they are faced with a bear lets say, will they run? You truly don’t know your character until you write them enough, which could make the beginning of your book sound weird because this isn’t how your character sounds like.
So, do little writing exercises. Like, maybe talk to the characters in your head, or write a diary in your character's POV. This will help you get to know them well and help them come to life in your book.