How do I even begin writing my first book?

So I’ve wanted to write a book for a long time now and I have a pretty good idea of the story and characters (there are a few characters that I haven’t exactly figured out yet, and while I’ve figured out the big story beats I’ve yet to fill in the cracks). So I’m wondering how other people approach starting the writing process. Should I wait until I’ve figured out every single character and story beat until I start writing? Should I just jump into it and figure it out from there? Is there a good way to structure the planning process or is that not really needed? Ofc different things are gonna work for different people but I’m just curious how other people go about doing this