Select a writing tool--your computer, pen or pencil--and then find a quiet place where you can write without distraction. Use a timer and set it to at least ten minutes and begin writing. To write a book you need to learn to write for more than ten minutes at a time, but this a good way to get yourself used to the process of free writing.
Forget about using correct grammar or punctuation. Don't worry about spelling or finding just the right word just keep writing without stop for about ten to twenty minutes. To write your book, you need to have a certain amount of words you write each day, so give yourself a daily or weekly amount of words to write and work to reach that until you complete your book.
Continue writing even if something doesn't make sense. Don't stop to make corrections or cross anything out. You can do corrections or rewrites after you've finished your book. Just write and let your imagination go until your book is complete.