Once you nail down what kind of book you want to end up with, you can get busy writing. The obvious way to go about this is to shut the door to the world, write whatever you want and worry about editing down the line. Show your friends and family when you want to, but otherwise, do it however you like. You don't have a publisher to worry about, so you can really write however you want to.
To many self-publishers, this doesn't work very well -- it's too unstructured, and they get lost without somebody to bounce ideas off of. One solution is to hire a freelance developmental editor. A developmental editor serves the same basic function as the editor you would work with at a publishing house -- you can show them drafts and outlines, and they can make edits to improve the book. The difference of course is that what you say is the last word, rather than the other way around. Ideally, the main thing they bring is expertise in book publishing -- a developmental editor should be somebody who knows how to build a good book.
To find a developmental editor, check the Literary Marketplace
The price of a developmental editor goes in your total budget for the book. Depending on how you work, it may save you enough of your own time to make it a worthwhile expense.
You may also want to bring in additional professional help. In the next section, we'll look at the other people you might need on your payroll.