How do I start writing? There are as many answers as there are authors, everyone does it differently. In a sense, anything I write was started years and years ago. Everything I read influences what I write, and I read a lot. That's a pretty diffuse beginning, though. Any specific book-length writing project starts about six months before I start writing the actual story. For NaNoWriMo novels, written in November, that means I start the process about now, late April or early May.
I like to do these early steps with pen on paper. That's just a personal choice, but the flexibility of formatting and the act of physically writing it seems to help the formative process. The initial notes are very, very general. My stories are character-driven (rather than plot-driven), so I'll jot down ideas about who my character(s) will be. My stories also tend to put ordinary people in extraordinary circumstances (sometimes the circumstances are external, sometimes something about the person), so I'll jot down ideas of the circumstances, too. I'll also take note of any common threads in the books I've recently enjoyed reading. You can see the book with some of the notes from Uncommon Counsel in the picture. That and the caffeine, of course.
Some of my earliest notes for Uncommon Counsel were "First person story of a modern character led into a fantasy experience. A character with admitted mental health issues so it is not clear whether the fantastic is real or a schizophrenic delusion -- and does it matter? The guides direct the protagonist to acts of kindness and charity. The delusions (or these delusions) are benign if not benevolent. Other guides will be less so, but the protagonist will have the choice which to follow." If you've read the book, you can see how that led to Uncommon Counsel, but I really had no firm ideas of any details at that point.
Another aspect of these early notes is that they are full of questions. Don't be afraid to leave unanswered questions. When you get stuck, go back and read your earlier notes, and take a stab at answering them. Also, don't be afraid to go down what may be blind alleys. This is the time to explore things. There's absolutely no rule that everything you write here finds its way into the finished story -- in fact it's probably terrible if it all does. This isn't time wasted, this is how your brain winnows out the chaff to find the true kernels of story. And that chaff may end up blowing into your next story where it is a perfect fit.
Typically I'll have at least 20 hand-written pages (in a 6" x 9" notebook) of notes by the time I'm ready to start the actual writing. It may be double that or more. The early notes are not much use other than showing how I got to my story. The later pages will have increasingly detailed notes -- minor characters, timelines, details of setting, research for plot points, etc. How do you know when to start writing the actual story? Well if you do NaNoWriMo, the answer is easy -- midnight on November 1! But if you don't have that fixed schedule, I think the answer is to delay it until you just about can't stand not to write. You want to have a built up pressure of storytelling to get things going over that initial hump. The first few thousand words should feel like a relief when you finally write them. If you're not that excited yet, then keep planning! Some stories take years to develop.
It's also perfectly ok to have several different stories in this planning phase at the same time. Work on the one that is speaking to you at the moment. If you go back and forth, that's fine. As you get into finer and finer detail, one will take precedence and that will be the one to start. Save the notes for the other stories for your next book. There's no right length of time a story takes to develop. It may leap fully-formed into your mind, demanding to be let out, or it may stew for years before it finally settles into something that demands to be written. Both of those are ok. For me, six months seems to be a good planning time, but not all stories follow that average.
As I said, my stories are character driven, not plot driven. When I start writing I may have little idea of anything beyond the broad strokes of a plot. What I do want to have is a pretty detailed description of my main characters, including backstory, strengths and weaknesses, character quirks and mannerisms, and what it is I want them to do or learn in the story. I like my characters to grow, and they typically all have something they need to learn. I want to know what that is so I can be sure they experience what they need to achieve that. I may have a few samples of the character's voice, too. Something written from their perspective, as they would narrate it. Not anything that will appear in the story, just something to let me know how they speak, how they think. This is especially important if you are writing a first-person story, as that character's voice will be how the whole story is related.
If you know details of the plot, a timeline can be really helpful. Notes about the settings are really valuable, too. And of course do your research before you start writing (though you'll doubtless need to do more once you start). Before I ever started writing Uncommon Counsel, I'd read a lot about schizophrenia, both from a patient's and a provider's viewpoint. I'd also researched legal ethics and how breaches are punished (specifically in Colorado).
If this sounds like a lot of work, well, it is. But it's a lot of fun, too. Writing a book is often described in terms of giving birth. And the actual writing of the first draft is like going through labor (at least from my male perspective). The planning phase is all the fun leading up to getting pregnant, and then the increasing pressure of pregnancy until you feel like you might explode if you don't give birth soon. Editing, of course, is raising the child until it finally leaves home as a published book. So how do you start writing? Get out there and fool around!