I've written a book, and written it again, and re-written it, then done some editing and a little revising, then gone over it again... Every now and then in the process I think it's done and send out a stack of query letters to agents and get back mostly form rejections, with the occasional hand-written note. I'm hope to send out another batch soon.
I've worked on mine pretty steadily over the past 5 1/2 years, with a few months off here and there. It takes a lot longer than I thought it would. I mean, you're always hearing about some whizz kid who churns out a few chapters then gets a 6-figure deal a week later, but the reality isn't like that for most of us. They say that it takes a long time to create an overnight success.
I could go on about this forever. Basically, you have to start with writing the first draft. If you're anything like me, you'll start at least half a dozen first drafts (maybe you've already done this?) before some idea settles in long enough to get to the end of the story. Outlines and character sketches can help.
Most writing books recommend setting asside time to write every day at the same time, in the same place, half an hour to two hours, preferably in the morning. You just start by putting words on paper, one after the other. If you do that for a year, you'll have a lot of words. Whether it's a novel (or a good novel) is another question, but it's a good start.
You can also cram the writing into bits of free time, whenever you get those. There's a new book called Writer Mama which I was looking at in the bookstore today. It's mostly focused on non-fiction, magazine articles, etc. There are dozens, maybe hundreds more which might be called: So Ya Want To Write A Book, Eh?
It also helps to join a writers' group, preferably locally, but on-line is easier.
I'm sure I'll think of more in a minute.