In the course of writing a fast first draft, you will learn not to be afraid of words. Your command over words will naturally increase and the more you learn about the words, the more you learn about writing. In fact, you're learning how to work. You know you can just throw words onto a page and see if they will stick. This will also make it easier for you, when you begin the editing process, to trash pages of work if you have to. Writers are usually hesitant to cut the words they've struggled so hard to create. But you won't have this difficulty because you'll know: you wrote these words once--you can write more!
How Fast is Fast?
The speed with which you finish your first draft will depend on you. Some people can pound out a draft in a month and for others that's just not possible. Maybe you can do a few pages a day, shooting to complete a draft in six months. The idea is to move along, writing as fast as you can, but it's more important to be in the flow. You want to feel as though you are a storyteller or you're giving a presentation and you are pouring your words out to an audience sitting right in front of you. Pretend you're on a bicycle: to stay upright, you must continually pedal and stay in motion. Do this and you will ride the flow all the way to your book's completion.