I write books as a hobby. I have one out now, and am writing the sequel.
The best advice I can give is to just sit down and write. Set aside some time every day. Don't fall in the habit of waiting for inspiration. Sometimes you just have to make yourself write - particularly after the first couple weeks, when the novelty of writing a book starts to wear off.
Once you have an outline, and hence, a sense of where you're going with the draft, you should be well on your way!
I wrote a blog post about inspiration, muses, and their unreliability a couple months ago. link
My editor for my first book has some good advice on a variety of writing issues link
(scroll down to writing).
If I can think of more advice, I'll pass it along. There's a lot of conflicting advice out there, too, so keep that in mind.