Writing a book is easy when you know how to do it - like most things, the first time you try something it seems like a big deal and yet when you have finished, it becomes clear how easy it was. Isn't hindsight a great thing!
First decide on your time frame - if you want to get this book out by Christmas and it's already September, then you'll need to seriously rush through these steps a lot more, but if you have more time to play with this wont necessarily mean getting your book out any faster. Once you set a time limit for yourself, break it down into three parts. First draft, Second draft and then production. Your first draft may only be half the contents that will end up being in the finished book, but it's a great way to start getting the information assembled and out of your note book or head and onto a manuscript.